Hey guys,
Found something out last night.
I had the Acid gat and we were collecting skulls when the Blunder Gat came out of the Warden's desk. I wanted to test if I could have two so I picked it up. Success! I had an Acid Gat and a Blunder Gat. Then I wanted to see if I could carry 2 Acid Gats so I upgraded that one as well... Unfortunately not.
If you upgrade the Blunder Gat whilst having an Acid Gat, when you pick up the new weapon your old one will disappear. Shame because that would have been awesome... for a while