As per FINANCIAL INFORMATION EXCHANGE (FIX) protocol PossDupFlag (tag 43): indicates possible retransmission of message with this sequence number valid value:

Y = Possible duplicate

N = Original transmission

PossResend (tag 97): Indicates that message may contain information that has been sent under another sequence number.

Valid Values:

Y=Possible resend

N=Original transmission

• In Simple terms PossDup is when you are resending a message and PossResend is when you are sending a new message with same DATA you have already sent in a previous message.
• Let’s consider below use case for clarity with PossDup, you can send out an order with Sequence number of 10. Then you send another message with a number of 11. For some REASON the counter party did not receive message 10 and will request a resend. You will then resend this same message with a sequence number of 10 marking tag 43=Y.
• In case of PossResend, you may send out an order, again let's say with a sequence number of 10. After 5 seconds, you have not gotten an acknowledgement. So you may decide to try again because maybe the counterparty didn't receive or got the first time. So you will send out a message with a different sequence number like 11, which will contain all of the same data message number 10 did. You will then mark it as a PossResend. So this is saying you have already sent this order before, which Counter party may or may not have processed. 
• While handling of PossDup is dictated by session logic, PossResend are dictated by business logic (e.g. Reject an Order if ClOrdID is duplicate etc). This is because it is up to the business LAYER to determine if a PARTICULAR business object has been processed (by CHECKING the order id for instance).

As per FINANCIAL INFORMATION EXCHANGE (FIX) protocol PossDupFlag (tag 43): indicates possible retransmission of message with this sequence number valid value:

Y = Possible duplicate

N = Original transmission

PossResend (tag 97): Indicates that message may contain information that has been sent under another sequence number.

Valid Values:

Y=Possible resend

N=Original transmission