diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs b/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs
index 9261dd2b..d81bf796 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs
@@ -131,6 +131,17 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         {
             if (_alarmConditions.TryGetValue(alarmNodeId, out var condition))
             {
+                // T20 delta-gate: read the node's CURRENT live condition state FIRST (before projecting
+                // the incoming snapshot onto it), then decide fire-vs-suppress by comparing the incoming
+                // snapshot to that current state. We gate against the NODE's state, NOT a "last written"
+                // cache, because an inbound client ack the SDK applied (OnAcknowledge returned Good →
+                // SDK mutated AckedState + auto-fired its own event) NEVER passed through this method, so a
+                // last-written cache would be stale and wrongly report a delta. By the time the engine
+                // re-projects that ack here, the node already holds the acked state → no delta → suppress.
+                var current = ReadConditionDelta(condition);
+                var incoming = ToConditionDelta(state, condition);
+                bool fire = ShouldFireConditionEvent(current, incoming);
+
                 // EnabledState / AckedState / ActiveState are mandatory children — always present after
                 // Create. Confirm + Shelving are optional Part 9 children: T14's real-server finding is
                 // that Create auto-builds them for our subtypes, but a base AlarmConditionState (or a
@@ -166,12 +177,17 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                 condition.Time.Value = sourceTimestampUtc;
                 condition.ReceiveTime.Value = sourceTimestampUtc;
 
-                // T16 — fire a real Part 9 condition event for THIS engine-driven transition. The
-                // ScriptedAlarmHostActor only calls WriteAlarmCondition on a REAL state transition
-                // (Emission != None/Suppressed), so every call corresponds to exactly one transition →
-                // fire exactly one event. ReportConditionEvent stamps a fresh EventId, ClearChangeMasks,
-                // and ReportEvent — all still under this lock.
-                ReportConditionEvent(condition, sourceTimestampUtc);
+                // T20 — fire a real Part 9 condition event ONLY when this projection is a genuine state
+                // change (the delta-gate decided above, against the node's pre-projection state). A
+                // genuine engine-driven transition (alarm goes active/clear, severity bucket shifts, an
+                // engine-side ack, etc.) differs from the node's current state → fire. The re-projection
+                // of a client ack the SDK already applied equals the node's current state → no delta →
+                // suppress, so we don't double-emit (E2 from the SDK + E3 from here). ReportConditionEvent
+                // stamps a fresh EventId, ClearChangeMasks, and ReportEvent — all still under this lock.
+                if (fire)
+                {
+                    ReportConditionEvent(condition, sourceTimestampUtc);
+                }
                 return;
             }
 
@@ -206,14 +222,16 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     ///         materialise) — no branch creation here.
     ///     </para>
     ///     <para>
-    ///         <b>Double-emit note (for T17).</b> This helper fires ONLY for ENGINE-DRIVEN (outbound)
-    ///         transitions routed through <see cref="WriteAlarmCondition"/>. T17's inbound
-    ///         Acknowledge/Confirm will go through the SDK's own <c>OnAcknowledgeCalled</c>, which already
-    ///         auto-fires a condition event (<c>ReportStateChange</c>) on a successful ack. These don't
-    ///         overlap today (T17 isn't built), but once it lands an engine-driven re-projection of an ack
-    ///         the SDK has ALREADY auto-emitted could double-emit the same logical transition — to be
-    ///         reconciled in T17 (either suppress the SDK auto-fire, or skip the engine re-projection for
-    ///         transitions that originated from an inbound method call).
+    ///         <b>Double-emit note (resolved by delta-gate).</b> An inbound client Acknowledge/Confirm
+    ///         goes through the SDK's own handler, which (after T18's gate returns Good) applies the acked
+    ///         state to the node and auto-fires its own condition event (E2) — directly on the node,
+    ///         BYPASSING <see cref="WriteAlarmCondition"/>. The engine then re-projects that same logical
+    ///         transition through <see cref="WriteAlarmCondition"/>, which would otherwise fire a second
+    ///         event (E3). <see cref="WriteAlarmCondition"/>'s delta-gate suppresses E3: it compares the
+    ///         incoming snapshot against the NODE's CURRENT state, and because the SDK has ALREADY
+    ///         pre-applied the inbound-ack state, the re-projection is a no-delta no-op (no fire). Genuine
+    ///         engine-driven transitions still differ from the node's current state, so they fire here as
+    ///         before.
     ///     </para>
     /// </summary>
     /// <param name="alarm">The materialised condition whose new state has already been projected; must be non-null.</param>
@@ -260,6 +278,84 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         }
     }
 
+    /// <summary>
+    ///     The gate-relevant slice of a Part 9 condition's state — exactly the fields that drive a
+    ///     condition event AND that an <see cref="AlarmConditionSnapshot"/> can change. As a record, two
+    ///     instances compare by value, so <see cref="ShouldFireConditionEvent"/> is a plain inequality.
+    ///     <para>
+    ///         <b>Severity</b> is stored as the MAPPED <see cref="EventSeverity"/> bucket (a
+    ///         <see cref="ushort"/>) — the same value the node holds after <c>SetSeverity</c> — so two
+    ///         raw severities that fall in the same bucket are correctly treated as "no change". The
+    ///         <b>Shelving</b> kind is read back from / mapped to the SDK's shelving state machine so the
+    ///         live node and the snapshot compare on the same 3-way (Unshelved/OneShot/Timed) axis.
+    ///     </para>
+    /// </summary>
+    internal readonly record struct AlarmConditionDelta(
+        bool Active,
+        bool Acknowledged,
+        bool Confirmed,
+        bool Enabled,
+        AlarmShelvingKind Shelving,
+        ushort MappedSeverity,
+        string Message);
+
+    /// <summary>Decide whether a <see cref="WriteAlarmCondition"/> projection is a genuine state change
+    /// (and so should fire a Part 9 condition event) by comparing the node's pre-projection state to the
+    /// incoming snapshot. Pure + value-based so it's unit-testable in isolation: returns <c>true</c> iff
+    /// any gate-relevant field differs. An inbound client ack the SDK already applied makes
+    /// <paramref name="current"/> == <paramref name="incoming"/> ⇒ <c>false</c> (suppress the re-projected
+    /// double-emit); a genuine engine-driven transition differs ⇒ <c>true</c> (fire).</summary>
+    /// <param name="current">The node's current (pre-projection) gate-relevant state.</param>
+    /// <param name="incoming">The incoming snapshot's gate-relevant state.</param>
+    /// <returns><c>true</c> to fire a condition event; <c>false</c> to suppress (no delta).</returns>
+    internal static bool ShouldFireConditionEvent(AlarmConditionDelta current, AlarmConditionDelta incoming) =>
+        current != incoming;
+
+    /// <summary>Read the gate-relevant slice off the LIVE condition node. Mandatory children
+    /// (Active/Acked/Enabled) are always present; Confirmed/Shelving are optional and null-guarded
+    /// (a leaner child set ⇒ treat as the unset default). Severity is read as the already-mapped
+    /// <see cref="EventSeverity"/> bucket the node stores, and shelving is mapped from the shelving
+    /// state machine's CurrentState so it lines up with <see cref="ToConditionDelta"/>.</summary>
+    private static AlarmConditionDelta ReadConditionDelta(AlarmConditionState condition) => new(
+        Active: condition.ActiveState?.Id?.Value ?? false,
+        Acknowledged: condition.AckedState?.Id?.Value ?? true,
+        Confirmed: condition.ConfirmedState?.Id?.Value ?? true,
+        Enabled: condition.EnabledState?.Id?.Value ?? true,
+        Shelving: ReadShelvingKind(condition),
+        MappedSeverity: condition.Severity?.Value ?? (ushort)0,
+        Message: condition.Message?.Value?.Text ?? string.Empty);
+
+    /// <summary>Build the gate-relevant slice from the incoming snapshot, normalising the two fields that
+    /// the node stores in a derived form: Severity is run through <see cref="MapSeverity"/> so it matches
+    /// the bucket the node holds (the projection calls <c>SetSeverity(MapSeverity(...))</c>), and an
+    /// optional Confirmed/Shelving that the node can't actually hold (missing child) is folded to the
+    /// node's read-back default so it never spuriously registers as a delta.</summary>
+    private static AlarmConditionDelta ToConditionDelta(AlarmConditionSnapshot state, AlarmConditionState condition) => new(
+        Active: state.Active,
+        Acknowledged: state.Acknowledged,
+        // If the node has no ConfirmedState child, the projection is a no-op there; mirror the node's
+        // read-back default (true) so a snapshot Confirmed value can't create a phantom delta.
+        Confirmed: condition.ConfirmedState is not null ? state.Confirmed : true,
+        Enabled: state.Enabled,
+        // Likewise for shelving: without a ShelvingState child the projection can't apply, so fold to the
+        // node's read-back default (Unshelved).
+        Shelving: condition.ShelvingState is not null ? state.Shelving : AlarmShelvingKind.Unshelved,
+        MappedSeverity: (ushort)MapSeverity(state.Severity),
+        Message: state.Message ?? string.Empty);
+
+    /// <summary>Map the live shelving state machine's CurrentState back to our 3-way
+    /// <see cref="AlarmShelvingKind"/> by matching its well-known Part 9 state object id. Any node without
+    /// a shelving sub-state machine (or an unrecognised/unset state) reads as
+    /// <see cref="AlarmShelvingKind.Unshelved"/> — the same value <see cref="ToConditionDelta"/> folds an
+    /// unsupported shelving snapshot to, so the two stay comparable.</summary>
+    private static AlarmShelvingKind ReadShelvingKind(AlarmConditionState condition)
+    {
+        var stateId = condition.ShelvingState?.CurrentState?.Id?.Value as NodeId;
+        if (stateId == ObjectIds.ShelvedStateMachineType_OneShotShelved) return AlarmShelvingKind.OneShot;
+        if (stateId == ObjectIds.ShelvedStateMachineType_TimedShelved) return AlarmShelvingKind.Timed;
+        return AlarmShelvingKind.Unshelved;
+    }
+
     /// <summary>
     ///     Materialise a real OPC UA Part 9 <see cref="AlarmConditionState"/> node under its equipment
     ///     folder so clients can browse it as a proper condition (and subscribe to its events). The node
@@ -339,9 +435,10 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
             // routing; these delegates are the veto/permission seam the SDK invokes BEFORE applying the
             // state change. Each gates on the caller's AlarmAck role (fails closed) and, when allowed,
             // routes a mapped AlarmCommand to the engine via AlarmCommandRouter, then returns Good so the
-            // SDK applies its node state + auto-fires its own event.
-            // T20: the engine re-projects that same logical transition through WriteAlarmCondition, which
-            // also fires — the resulting double-emit is de-duped in a later task (T20), NOT here.
+            // SDK applies its node state + auto-fires its own event (E2).
+            // T20: the engine re-projects that same logical transition through WriteAlarmCondition; its
+            // delta-gate (compares against the node's current state, which the SDK already pre-applied)
+            // sees no change and suppresses the would-be second event (E3) — so no double-emit.
             alarm.OnAcknowledge = (context, condition, _, comment) =>
                 HandleAlarmCommand(context, condition, "Acknowledge", comment, unshelveAt: null);
             alarm.OnConfirm = (context, condition, _, comment) =>
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/SdkAddressSpaceSinkTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/SdkAddressSpaceSinkTests.cs
index 07215109..38030fb2 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/SdkAddressSpaceSinkTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/SdkAddressSpaceSinkTests.cs
@@ -354,6 +354,127 @@ public sealed class SdkAddressSpaceSinkTests : IDisposable
         await host.DisposeAsync();
     }
 
+    /// <summary>T20 — the inbound double-emit is suppressed by the delta-gate. We simulate the REAL
+    /// inbound sequence: a client Acknowledge whose T18 gate returned Good causes the SDK to apply the
+    /// acked state to the node and auto-fire its OWN event (E2) WITHOUT going through WriteAlarmCondition.
+    /// We reproduce that by applying the acked state directly onto the live AlarmConditionState the way
+    /// the SDK would. THEN the engine re-projects the same logical transition through WriteAlarmCondition
+    /// with the matching snapshot — and because that snapshot equals the node's current state, the
+    /// delta-gate must fire NO event (no E3). We prove "no event fired" by asserting the condition's
+    /// EventId is UNCHANGED across the WriteAlarmCondition call (ReportConditionEvent always restamps a
+    /// fresh GUID EventId when it fires — same probe the T16 event test uses).</summary>
+    [Fact]
+    public async Task WriteAlarmCondition_suppresses_event_when_snapshot_equals_node_current_state()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+        var sink = new SdkAddressSpaceSink(nm);
+
+        sink.EnsureFolder("eq-ack", parentNodeId: null, displayName: "Equipment Ack");
+        sink.MaterialiseAlarmCondition("alm-ack", "eq-ack", "HighTemp", "OffNormalAlarm", severity: 700);
+
+        var condition = nm.TryGetAlarmCondition("alm-ack");
+        condition.ShouldNotBeNull();
+
+        // Drive the alarm active+unacked through the engine path (a genuine transition → fires).
+        sink.WriteAlarmCondition("alm-ack", Snapshot(active: true, acknowledged: false, message: "active"), DateTime.UtcNow);
+        condition!.ActiveState.Id.Value.ShouldBeTrue();
+        condition.AckedState.Id.Value.ShouldBeFalse();
+
+        // === Simulate the SDK-applied inbound Acknowledge (E2) ===
+        // After T18's gate returns Good, the SDK applies the acked state to the node and auto-fires its
+        // own event — directly on the node, BYPASSING WriteAlarmCondition. Reproduce that node mutation.
+        lock (nm.Lock)
+        {
+            condition.SetAcknowledgedState(nm.SystemContext, true);
+            condition.Message.Value = new LocalizedText("active");
+            condition.ClearChangeMasks(nm.SystemContext, includeChildren: true);
+        }
+        condition.AckedState.Id.Value.ShouldBeTrue();
+
+        // Capture the EventId AFTER the SDK-applied ack but BEFORE the engine re-projection.
+        var beforeReProject = (byte[]?)condition.EventId.Value?.Clone();
+
+        // === Engine re-projects the SAME acked transition through WriteAlarmCondition (would-be E3) ===
+        // Snapshot equals the node's current state (active, acked, message "active") ⇒ delta-gate sees
+        // no change ⇒ NO event fires.
+        sink.WriteAlarmCondition("alm-ack", Snapshot(active: true, acknowledged: true, message: "active"), DateTime.UtcNow);
+
+        var afterReProject = (byte[]?)condition.EventId.Value?.Clone();
+        // EventId is UNCHANGED ⇒ ReportConditionEvent did NOT run ⇒ E3 suppressed.
+        afterReProject.ShouldBe(beforeReProject);
+
+        // The projection still applied (state is intact) — only the redundant event was suppressed.
+        condition.ActiveState.Id.Value.ShouldBeTrue();
+        condition.AckedState.Id.Value.ShouldBeTrue();
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>T20 — genuine engine-driven transitions still fire exactly one event, and a second
+    /// IDENTICAL WriteAlarmCondition (no delta vs the node's now-current state) fires zero more. We use
+    /// the EventId-changed-on-fire probe: a fire restamps a fresh GUID EventId, a suppress leaves it
+    /// untouched.</summary>
+    [Fact]
+    public async Task WriteAlarmCondition_fires_on_delta_and_suppresses_identical_reprojection()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+        var sink = new SdkAddressSpaceSink(nm);
+
+        sink.EnsureFolder("eq-delta", parentNodeId: null, displayName: "Equipment Delta");
+        sink.MaterialiseAlarmCondition("alm-delta", "eq-delta", "HighTemp", "OffNormalAlarm", severity: 700);
+
+        var condition = nm.TryGetAlarmCondition("alm-delta");
+        condition.ShouldNotBeNull();
+
+        var beforeFirst = (byte[]?)condition!.EventId.Value?.Clone();
+
+        // Genuine transition: snapshot (active, unacked) differs from the materialise state
+        // (inactive, acked) ⇒ delta ⇒ fires exactly one event (EventId changes).
+        sink.WriteAlarmCondition("alm-delta", Snapshot(active: true, acknowledged: false, message: "active"), DateTime.UtcNow);
+        var afterFirst = (byte[]?)condition.EventId.Value?.Clone();
+        afterFirst.ShouldNotBeNull();
+        afterFirst!.ShouldNotBe(beforeFirst); // fired
+
+        // Identical re-projection: snapshot now EQUALS the node's current state ⇒ no delta ⇒ 0 more
+        // events (EventId unchanged from the first fire).
+        sink.WriteAlarmCondition("alm-delta", Snapshot(active: true, acknowledged: false, message: "active"), DateTime.UtcNow);
+        var afterSecond = (byte[]?)condition.EventId.Value?.Clone();
+        afterSecond.ShouldBe(afterFirst); // suppressed
+
+        // A FURTHER genuine transition (clear) differs again ⇒ fires once more.
+        sink.WriteAlarmCondition("alm-delta", Snapshot(active: false, acknowledged: true, message: "cleared"), DateTime.UtcNow);
+        var afterThird = (byte[]?)condition.EventId.Value?.Clone();
+        afterThird.ShouldNotBe(afterSecond); // fired
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>T20 — direct unit test of the pure fire-vs-suppress decision seam
+    /// (<see cref="OtOpcUaNodeManager.ShouldFireConditionEvent"/>). Equal states ⇒ suppress; any single
+    /// field differing ⇒ fire. This pins the gate logic independent of the booted server.</summary>
+    [Fact]
+    public void ShouldFireConditionEvent_fires_only_on_a_field_delta()
+    {
+        var baseState = new OtOpcUaNodeManager.AlarmConditionDelta(
+            Active: true, Acknowledged: false, Confirmed: true, Enabled: true,
+            Shelving: AlarmShelvingKind.Unshelved, MappedSeverity: 100, Message: "m");
+
+        // Equal ⇒ suppress (this is the inbound double-emit case in pure form).
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState).ShouldBeFalse();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { }).ShouldBeFalse();
+
+        // Each single-field difference ⇒ fire.
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { Active = false }).ShouldBeTrue();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { Acknowledged = true }).ShouldBeTrue();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { Confirmed = false }).ShouldBeTrue();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { Enabled = false }).ShouldBeTrue();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { Shelving = AlarmShelvingKind.Timed }).ShouldBeTrue();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { MappedSeverity = 900 }).ShouldBeTrue();
+        OtOpcUaNodeManager.ShouldFireConditionEvent(baseState, baseState with { Message = "other" }).ShouldBeTrue();
+    }
+
     /// <summary>Builds a test <see cref="AlarmConditionSnapshot"/> with sensible defaults so each call
     /// site only specifies the fields it cares about.</summary>
     private static AlarmConditionSnapshot Snapshot(