I got a Mega Disenchanter as a drop and it has the same sprite as the Legendary Disenchanter. It has no function or description on the equipment screen, but it's described as a backpack on the store screen. Very weird.
Weapons respawn if you leave the room and go back in. If you find a room with a good gun, you can easily equip yourself with 4 copies of it. This should probably be fixed...