So after the raid mode patch apparently Single player got broken. Special weapons you recieve as rewards can potentially duplicate themselves several times. I'm not sure what causes it, but I think repeatedly dying before looking at them in your workbench has something to do with it. Of course this is only a theory.
Anyway you can read about reports of it here http://www.gamefaqs.com/boards/835626-resident-evil-revelations-2/71706914
As you can see, the glitch can be unstable at times. I've experimented with it before and my trials have got different results. Apparently, the order of moving weapons around in the workbench after they've been duplicated can have different effects. You can have weapons and parts deleted (if a weapon disappears with a part on it.), you can duplicate custom parts, and occasionally the weapon might show up as a blank space and if it's moved, it disappears. Doing a Retry without getting to a checkpoint can freeze the game on a black screen after doing this.
You can also equip multiples of the same weapon, but there's really no point outside of the Chicago Typewriter. I haven't been able to duplicate weapons found in the campaign yet. The only weapons I've had duplicated are EX weapons and the typewriter.
There is one perk to this glitch however. If you equip a part to one copy of a weapon and switch the duplicates between the workbench and weapon slots, you can duplicate the part that's in the parts slot! Though if you do it wrong and have a weapon with a blank slot, you'll end up losing the part on the other copy. So far, I've only been able to do this with the Chicago Typewriter. That's unfortunate because it means any parts not compatible with the machine pistol can't be duplicated. I now have Damage 4 on most if not all of my weapons and the best capacity part for my respective character.
If a weapon or part disappears, don't beat the level. If it's a part that got deleted, just hit retry and start over. If a weapon got deleted, run to the next checkpoint, and select quit and restart the episode from the episode select. If you do get a game freeze, it will usually be on the load screen. The screen will be completely black with no hints or loading icon. I've tested it a few times from my trial and error and it should be safe to turn off your console.
I can confirm that this glitch is in the 360 and PS3 versions. As for PC, Xbox One, and PS4, I dunno. If anyone would like to confirm this glitch for the three consoles I'm unsure of, please feel free. The issue seems to have started with the Raid mode compatibility patch.