Duplicate EDI issue in TN

We are facing a peculiar duplication problem. During our maintenance schedule, our Proxies are up, but our Integration Servers are down for around 3 hours. Within that time period, if any EDIINT is hitting the Proxy, then the Sender do not get any MDN back as our IS are down, and hence they repost them. But somehow all those transactions are getting stored somewhere, and when the IS is up, they are posted to IS, resulting in duplicates.
We are running on 7.1. Any help in this regard is highly appreciated.

It’s weird.But As a work around may be you can set for check for duplicate document (via built-in or custom service) in the rule EDIINT ProcessMessage or custom rules (Pre-Processing) and avoid changing EDIINT out of box rules.

And I don’t think transactions gets stored while IS is down…Do you have DMZ IS servers or its just proxy layer?