Avoid duplicate application listeners (proxy vs. proxy target)

In AbstractApplicationEventMulticaster.retrieveApplicationListeners,
despite best efforts to avoid it, unwrapped proxies (singleton targets)
can end up in the list of programmatically registered listeners. In
order to avoid duplicates, we need to find and replace them by their
proxy counterparts, because if both a proxy and its target end up in
'allListeners', listeners will fire twice.

Fixes #28283.

Author: kriegaex

spring-context/src/main/java/org/springframework/context/event/AbstractApplicationEventMulticaster.java

@@ -263,6 +263,24 @@ private Collection<ApplicationListener<?>> retrieveApplicationListeners(
 					if (supportsEvent(beanFactory, listenerBeanName, eventType)) {
 						ApplicationListener<?> listener =
 								beanFactory.getBean(listenerBeanName, ApplicationListener.class);
+
+						// Despite best efforts to avoid it, unwrapped proxies (singleton targets) can end up in the
+						// list of programmatically registered listeners. In order to avoid duplicates, we need to find
+						// and replace them by their proxy counterparts, because if both a proxy and its target end up
+						// in 'allListeners', listeners will fire twice.
+						ApplicationListener<?> unwrappedListener =
+								(ApplicationListener<?>) AopProxyUtils.getSingletonTarget(listener);
+						if (listener != unwrappedListener) {
+							if (filteredListeners != null && filteredListeners.contains(unwrappedListener)) {
+								filteredListeners.remove(unwrappedListener);
+								filteredListeners.add(listener);
+							}
+							if (allListeners.contains(unwrappedListener)) {
+								allListeners.remove(unwrappedListener);
+								allListeners.add(listener);
+							}
+						}
+
 						if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
 							if (retriever != null) {
 								if (beanFactory.isSingleton(listenerBeanName)) {