AbstractBeanFactory.markBeanAsCreated() reliably clears merged bean definition only once

Issue: SPR-14269

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

@@ -1503,10 +1503,13 @@ public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport imp
 	 */
 	protected void markBeanAsCreated(String beanName) {
 		if (!this.alreadyCreated.contains(beanName)) {
+			synchronized (this.mergedBeanDefinitions) {
+				if (this.alreadyCreated.add(beanName)) {
 					// Let the bean definition get re-merged now that we're actually creating
 					// the bean... just in case some of its metadata changed in the meantime.
 					clearMergedBeanDefinition(beanName);
-			this.alreadyCreated.add(beanName);
+				}
+			}
 		}
 	}