Two fields help with resending messages:

The PossDupFlag (43) 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 PossResend (97) is set to Y when reissuing a message with a new sequence number (e.g. resending an order).

The receiving application should process these messages as follows:

PossDupFlag - if a message with this sequence number has been previously received, ignore message, if not, process normally.
PossResend - forward message to application and determine if previously received (i.e. verify order id and parameters).

	Created by Thiago Pavesi about 10 years ago

Understading the inner works of FIX protocol

Anatomy of FIX Protocol (Intro)