src/share/classes/java/lang/ref/ReferenceQueue.java

*** 56,65 ****
--- 56,72 ----
  
      boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
          synchronized (r) {
              if (r.queue == ENQUEUED) return false;
              synchronized (lock) {
+                 // There is a race between this method and the polling: it may occur
+                 // that one thread just successfully removed a Reference from the queue,
+                 // making it inactive, and another thread calling this method on this
+                 // queue with the same Reference instance. Filter out those attempts.
+                 if (r.queue != this) {
+                     return false;
+                 }
                  r.queue = ENQUEUED;
                  r.next = (head == null) ? r : head;
                  head = r;
                  queueLength++;
                  if (r instanceof FinalReference) {