Fix Galaxy paging review findings

clients/go/mxgateway/galaxy.go

@@ -3,6 +3,7 @@ package mxgateway
 import (
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"time"
 
@@ -148,6 +149,7 @@ func (c *GalaxyClient) DiscoverHierarchy(ctx context.Context) ([]*GalaxyObject,
 	defer cancel()
 
 	var objects []*GalaxyObject
+	seenPageTokens := make(map[string]struct{})
 	pageToken := ""
 	for {
 		reply, err := c.raw.DiscoverHierarchy(callCtx, &pb.DiscoverHierarchyRequest{
@@ -162,6 +164,10 @@ func (c *GalaxyClient) DiscoverHierarchy(ctx context.Context) ([]*GalaxyObject,
 		if pageToken == "" {
 			break
 		}
+		if _, seen := seenPageTokens[pageToken]; seen {
+			return nil, fmt.Errorf("mxgateway: galaxy discover hierarchy returned repeated page token %q", pageToken)
+		}
+		seenPageTokens[pageToken] = struct{}{}
 	}
 	return objects, nil
 }