How to access the underlying default concurrent queue of a blocking collection?

I have multiple producers and a single consumer. However if there is something in the queue that is not yet consumed a producer should not queue it again. (unique no duplicates blocking collection that uses the default concurrent queue)

if (!myBlockingColl.Contains(item))
    myBlockingColl.Add(item)

However the blocking collection does not have a contains method nor does it provide any kind of TryPeek() like method. How can I access the underlying concurrent queue so I can do something like

if (!myBlockingColl.myConcurQ.trypeek(item)
  myBlockingColl.Add(item)

In a tail spin?