Make sure that no fileIds are replayed to the client (#12330)

Due to how ProxyReceivers are constructed and serialized, we don't want
to resend the ProxyReceivers for files that already are in the queue.
Either we needed to change the way ProxyReceivers behave, or just make
sure that they aren't resent. I chose the double-check method.

Change-Id: I7c4e866ba4287afd3e80329be3640335d4ff00af