Merge branch '6.1.x'

Author: rstoyanchev

spring-messaging/src/main/java/org/springframework/messaging/simp/broker/DefaultSubscriptionRegistry.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -305,6 +305,9 @@ private void addMatchedSubscriptionId(
 					return Collections.singletonList(subscriptionId);
 				}
 				else {
+					if (subscriptionIds.contains(subscriptionId)) {
+						return subscriptionIds;
+					}
 					List<String> result = new ArrayList<>(subscriptionIds.size() + 1);
 					result.addAll(subscriptionIds);
 					result.add(subscriptionId);

spring-messaging/src/test/java/org/springframework/messaging/simp/broker/DefaultSubscriptionRegistryTests.java

@@ -102,6 +102,16 @@ void registerSameSubscriptionTwice() {
 		MultiValueMap<String, String> actual = this.registry.findSubscriptions(createMessage(dest));
 		assertThat(actual).hasSize(1);
 		assertThat(actual.get(sessId)).containsExactly(subId);
+
+		// Register more after destinationCache populated through findSubscriptions,
+		// and make sure it's still only one subscriptionId
+
+		this.registry.registerSubscription(subscribeMessage(sessId, subId, dest));
+		this.registry.registerSubscription(subscribeMessage(sessId, subId, dest));
+
+		actual = this.registry.findSubscriptions(createMessage(dest));
+		assertThat(actual).hasSize(1);
+		assertThat(actual.get(sessId)).containsExactly(subId);
 	}
 
 	@Test