Leniently accept same singleton instance if implicitly appeared

Closes gh-34427

spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java

@@ -368,8 +368,18 @@ public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
 					}
 					afterSingletonCreation(beanName);
 				}
+
 				if (newSingleton) {
-					addSingleton(beanName, singletonObject);
+					try {
+						addSingleton(beanName, singletonObject);
+					}
+					catch (IllegalStateException ex) {
+						// Leniently accept same instance if implicitly appeared.
+						Object object = this.singletonObjects.get(beanName);
+						if (singletonObject != object) {
+							throw ex;
+						}
+					}
 				}
 			}
 			return singletonObject;

spring-beans/src/test/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistryTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -50,10 +50,15 @@ class DefaultSingletonBeanRegistryTests {
 		assertThat(beanRegistry.getSingleton("tb2")).isSameAs(tb2);
 		assertThat(tb2Flag.get()).isTrue();
 
-		assertThat(beanRegistry.getSingleton("tb")).isSameAs(tb);
-		assertThat(beanRegistry.getSingleton("tb2")).isSameAs(tb2);
-		assertThat(beanRegistry.getSingletonCount()).isEqualTo(2);
-		assertThat(beanRegistry.getSingletonNames()).containsExactly("tb", "tb2");
+		TestBean tb3 = (TestBean) beanRegistry.getSingleton("tb3", () -> {
+			TestBean newTb = new TestBean();
+			beanRegistry.registerSingleton("tb3", newTb);
+			return newTb;
+		});
+		assertThat(beanRegistry.getSingleton("tb3")).isSameAs(tb3);
+
+		assertThat(beanRegistry.getSingletonCount()).isEqualTo(3);
+		assertThat(beanRegistry.getSingletonNames()).containsExactly("tb", "tb2", "tb3");
 
 		beanRegistry.destroySingletons();
 		assertThat(beanRegistry.getSingletonCount()).isZero();