Ok, so I had 2 of the same treasure maps (Murkmire 1). I had 1 in my bank and 1 in my inventory.
Then I got a reward box containing another copy of the map. I tried to open the box, but of course it said I could not take it because there was already one in my inventory.
Fair enough, so I dug up the one from my inventory.
Then I opened the reward box again.
BUG! The game still said I could not take it because I already had one in my inventory.
I made 100% sure there was no Murkmire treasure map in my inventory anymore. There was just 1 sitting in the bank. No matter how hard I tried (relog etc), I could not
retrieve the treasure map from the reward box, because it said I already had one in my inventory.
Then I took the one from the bank (no complaints about having already one) and dug that up. Now it would let me retrieve the map from the reward box.
But the fact the bank was able to add it to my inventory and the box was not able to add it to my inventory clearly shows there is a bug there.