I have repeated this numerous times through the years and found it to be oddly controversial in ways that no one ever explains to me...fairly warned, be thee, says I...
I don't think most things we do in games have ANY moral ramifications when we play alone. You want to know if duping shells is okay? Ask yourself this: Do I feel cheated? Because you are the only one you can rob in that circumstance. If duping that shell makes you happy then may you have a million chests full of them in five minutes! If, on the other hand, you suddenly find that the effort-reward thing happening with video games goes to crap, you may want to consider that shells are only satisfying to you if you put in the hard work to get them legit. There's no moral absolutism to be found here. It's purely subjective. You have to know yourself and know how you feel about the game to have your answer.
THAT SAID! This does not apply online! Duping items in a server that orbids such exploits IS immoral. There's a certain competitive nature to multiplaye rmode. Resources are sort of limited and you have to each put in the work necessary to get them. It's a bit of a social contract that you violate when you cheat. It's a completely different scenario from duping an item in single player. It ultimately DOESN'T matter, it is just a game. But the part of the game that makes people happiest is how the social aspect influences the other areas of gameplay, so you really are guilty of a minor social violation when you cheat in multiplayer.
That's how I see it. Lots of people don't like what I just said and I don't know if you will either. But there it is!
Well said sir. I tip my hat to you.