A SIP UAS will retransmit the 2xx response multiple times until it receives the ACK request from the UAC. See the SIP RFC chapter "13.3 UAS Processing" for the gory details.
Most often this sort of problem happens when the ACK sent by the UAC is invalid and thus prevents the UAS from matching it to the INVITE transaction. ACK requests have special rules about their construction see 13.2.2.4 2xx Responses. To make things even more difficult the construction of the ACK request differs for 2xx and non-2xx final responses.