Inventory transfers between shells - if you switch to other shell and back you get 2x everything you had.

Let me guess... You have Levels (Weapon Levels) installed as well? This is causing issues atm, already reported on their side and some safty added on our side for the next release

Ok, thanks for fast response :D
edit:
Just checked, we don't have this mod

What, besides Weapon Levels, may cause this issue?

nothing else is known to, but as I said, sync tries to deduplicate these items if other mods fail to clean up (but the logic was not working in 7.0.2). Try using https://github.com/iChun/Sync/files/1890896/Sync-1.12.2-7.0.3BETA.zip and tell me if it fixes the issue (you have to unzip the zip file and replace your sync file with this)

Ok, now it copies inventory to shell on syncing, but when you go back to previous shell you don't get duplicate of all items