# UNDF: UNDF-2026-000000617
--- a/activemq-broker/src/main/java/org/apache/activemq/broker/region/Topic.java
+++ b/activemq-broker/src/main/java/org/apache/activemq/broker/region/Topic.java
@@ -21,6 +21,8 @@ import java.util.ArrayList;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Collections;
+import java.util.Set;
 import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.ConcurrentHashMap;

@@ -83,6 +85,8 @@ public class Topic extends BaseDestination implements Task {
     protected final CopyOnWriteArrayList<Subscription> consumers = new CopyOnWriteArrayList<Subscription>();
+    // O(1) membership guard — parallel to consumers list; updated under consumers monitor
+    private final Set<Subscription> consumerSet =
+        Collections.newSetFromMap(new ConcurrentHashMap<Subscription, Boolean>());

@@ -149,7 +153,7 @@ public class Topic extends BaseDestination implements Task {
                     boolean applyRecovery = false;
                     synchronized (consumers) {
-                        if (!consumers.contains(sub)){
+                        if (consumerSet.add(sub)){
                             sub.add(context, this);
                             consumers.add(sub);
                             applyRecovery=true;
@@ -165,7 +169,7 @@ public class Topic extends BaseDestination implements Task {
             } else {
                 synchronized (consumers) {
-                    if (!consumers.contains(sub)){
+                    if (consumerSet.add(sub)){
                         sub.add(context, this);
                         consumers.add(sub);
                         super.addSubscription(context, sub);
@@ -207,12 +211,14 @@ public class Topic extends BaseDestination implements Task {
         synchronized (consumers) {
-            removed = consumers.remove(sub);
+            removed = consumers.remove(sub);
+            if (removed) {
+                consumerSet.remove(sub);
+            }
         }

@@ -225,6 +231,7 @@ public class Topic extends BaseDestination implements Task {
             synchronized (consumers) {
                 consumers.remove(removed);
+                consumerSet.remove(removed);
             }

@@ -290,7 +297,7 @@ public class Topic extends BaseDestination implements Task {
                     synchronized (consumers) {
                         consumers.remove(subscription);
+                        consumerSet.remove(subscription);
                     }
                 }
                 synchronized (consumers) {
-                    if (!consumers.contains(subscription)) {
+                    if (consumerSet.add(subscription)) {
                         consumers.add(subscription);
                     }