Hello!
Today check Packet Sniffer in RB750Gr3, and found very big quantity of TCP Retransmission and TCP Dup ACK:How to deal with this?

Hey. Search for a CPU and ethernet interface load on a server side, also CRC errors on lines, check transit equipment, it's availability with icmp, udp. Is there any loss of traffic? What is the traffic and why it exists?

There isn't quite enough detail to confirm this in your post, but based on the screenshot, my first thought is you are getting duplicate packets 100% of the time. Furthermore, the duplicate packets are arriving extremely quickly, indicating that they aren't really retransmissions.

Check to see if you have the router mirroring packets to the same host you are capturing on. Check one of the retransmitted frames in Wireshark. It might have been received on UDP port 37008 (TZSP), or perhaps you have a switch port mirrored also.

Does not seem to be duplicate packets, as per screenshot, the sequence numbers changes. so I suspect packet loss between devices

+1 to CZFan.