PossDupFlag <43> field – FIXT 1.1 – FIX Dictionary
The PossDupFlag is set to Y when resending a message as the result of a session level event (i.e. the retransmission of a message reusing a sequence number).
The receiving application should process message with PossDupFlag = "Y" as follows:
if a message with this sequence number has been previously received, ignore message, if not, process normally.
N = Original transmission
Y = Possible duplicate