Old messages appearing multiple times when new message sent

I have a team of users running Spark 3.6.3 as their messenger, and OpenFire 3.8.2 as a server backend (Windows 2003 server). I am getting reports that sometimes when they send a message to a user, they instead see a message that was sent previously (sometimes days earlier) and the message repeats several times. I have had another separate messaging environment running the same software for several years, and this issue has never occurred.

I am assuming that this is related to the server - but I suppose it could just as well be the client that is doing it.

Has anyone seen this behavior before? Or can anyone recommend a good place to start troubleshooting?

Thanks,

-Steve Ballantyne