When my friend took items off the floating shelf, to me it looked like they were still there. But when I clicked to take these glitch items out, they just disappeared but didn't fortunately give me any duplicate items..

I had a similar issue locally when developing the shelf functionality. It has to do with the block state not syncing to the clients correctly. I'll look into how to fix this in multi-player.

Thanks!

This is fixed in 2.5.1, which was just published to CurseForge.

Thanks!