NakReceiverWindow: concurrent insertion may insert the same seqno multiple times

When concurrently inserting the same range of seqnos (e.g. 1-100) into the NakReceiverWindow by N threads (e.g. 10), we sometimes end up with duplicate messages for the same seqno ! This happens very rarely and only when many threads insert the same message !

This can be observed in DuplicateTest (testOOBMulticastsToAll3Senders()): sometimes the final list contains more than 10 elements.

I reproduced this by testing NakReceiverWindow in isolation, the test is NakReceiverWindowTest2.

I assume this is a regression introduced in 2.8 by moving insertions to and removal from xmit_table out of the write_lock scope in method NakReceiverWindow.add().