Do you know how often this duplicate response is being sent i.e. what is the time interval?
One of my users reported the exact same problem so I found this thread.
18.2.0 - Build:135435
An invite was sent to my user 2 weeks ago and she accepted.
Since then between 1 to 4 "Accepted" replies are sent to the original sender every day at random times.
In terms of checking the GWIA queue, am I suppose to check the 'send', 'defer', 'wpcsout' folders in \wpgate\gwia?
I checked these folders and there are just empty subfolders within these folders.
Should I delete/rename any of these and let GWIA recreate them?
Any other troubleshooting methods?
Do the users have an ActiveSync account which they use with some sort of mobile devices? Just a guess, but there are so many badly coded calendar apps around that i wouldn't be surprised if one of them would send an "accept" when changing the notification time. Or due to allowing the accept twice. Or 'cause it resends on every "open" followed by "finish".