No PossDup checks are run if the SeqNum is the expected next one.

There should be some error checking on the OrigSendingTime if the MsgSeqNum is correct and the PossDupFlag is set and a reject message sent if this fails. The code is there if the MsgSeqNum is less than expected but it is not run if the MsgSeqNum is as expected. See FIX 4.4 Vol 2 Test case 2f&g