two different packets with same seq

edit

I assume you're speaking about TCP, so if the first packet has expected SEQ (in-order), a client will:

• accept the first packet, advance sliding window and generate an ACK containing next expected SEQ;
• discard the second packet considering it out-of-order and in response one more time generate an ACK containing next expected SEQ (same as before).

The same behavior will continue until a client gets a packet with expected SEQ.