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[TCP Handshake]Server respond ack only instead of syn/ack

Hi Experts

I encountered an TCP handshake issue. Client initiated SYN(No.1 & 2) to server, however server responded an ACK(No.3) not SYN/ACK, but after that the server send a SYN/ACK(No.6) back to client.

According to pcap, it seems client retransmission the SYN till it RST this handshake. But somehow, the client try to re-establish the connection, and it worked normal as SYN , SYN/ACK , ACK.

My question is,

  1. What kind of situation will cause this issue happen?
  2. During handshake phase, what will client do if he received 2 ACKs ?

More Info:

  1. The dump is captured at server side interface.
  2. pcap file :
  3. Image :

Sorry for that I don't have point to upload here

Thank you,