Handle retries

We are using B2B server 4.0. I have a question regarding a 3A4 conversation. Our partner sends us a 3A4 (PO request), which our B2B server receives and processes. Our partner set the retries to ‘0’. i.e. they do not retry/resend the document if they did not receive the receipt ack for the first attempt. Everything worked fine so far. But, now they want to set the retry count to 3. But, our system does not handle the retries. For example, if the first attempt failed for some reason without even sending the receipt ack, and our partner sends the first retry with the same conversation id, we get out-of-bounds error. What is it i need to do to fix the problem? Should i clean the original conversation before processing the retry document? If so, how do i clean up the first conversation?
Any help is appreciated.