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 over 8 years ago

Understading the inner works of FIX protocol

Anatomy of FIX Protocol (Intro)