title: Concurrent Set Queue
body: <p>Maybe this is a silly question, but I cannot seem to find an obvious answer.</p>
  
  <p>I need a concurrent FIFO queue that contains only unique values. Attempting to add a value that already exists in the queue simply ignores that value. Which, if not for the thread safety would be trivial. Is there a data structure in Java or maybe a code snipit on the interwebs that exhibits this behavior?</p>