1. You can wear two rings that are named and enchanted identically if you have crafted them yourself at the altar or via sigil stone. You cannot wear two identical rings if they are ones that are 'pre enchanted' or already exist in the game.
Why? I suppose one could submit that wearing a pair of mundane rings is rather potent, or that some of the rings that exist in the game can carry several enchantments, whereas the rings you can make can only have one enchantment. Regardless though, that is just the way the game works.
2. I am not familiar with a 'Derrin' or a quest called 'The Abandoned Shop.' If you are sure you have the names correct, I suspect you are talking about a character and a quest from a mod perhaps?