When opening the Tacklebox on LAN with my friend, I managed to see his fishing rod, and I could upgrade it.

This has also allowed me to dupe a hook i placed into the tacklebox, by picking it up, and placing it down in the upgrade slot. I cannot really tell if my friend clicked the slot at the same time or so, but I ended up with 2 instances of the hook, and it wasn't a client dupe. after restarting we still had 2 hooks.

Maybe have the upgrade container only be accessible on a per client base ? like the crafting table.
The storage space could still be used as a chest and as a 'view for all'.

Thanks for your report!
It is intended behaviour. The dupe part of it is not, I'll have to look into that at some point.

The dupe was not reliably reproducible.