A few days ago I was checking some players inventories, and they started to dupe in a weird way. The hot bar seems to dupe once at a time, in a vertical manner, per opening of a player's inventory. It's hard to describe the situation better than that. I am running Purpur 1.20.1-2004, and was running the latest version of openinv.

I'm not able to reproduce this. Have you been able to get this to occur without other plugins? Are you in creative when the unexpected inventory behavior occurs?