Our current cheat policy says
A cheat in a multiplayer game, for the purposes of determining whether a question violates our cheating guideline, is a tool that uses out-of-game resources to give you an advantage that the other players in the game don't have.
Remember that questions that break the terms of service of the game in question are also off topic.
Questions covered by the first point are banned primarily because we don't want to give players an unfair competitive advantage. Questions covered by the second point are banned because, as LesssPop_MoreFizz said in a comment,
we're not in the business of enforcing other peoples TOS or NDA's, but the existence of said TOS and NDA's provide nice bright red lines for what are otherwise often fuzzy and highly disputed concepts.
Duplicating items in an MMO is an action that is likely against their rules, so we wouldn't accept it.
Duplicating items in Animal Crossing would be allowed, because it (apparently) doesn't require out-of-game resources, doesn't give anyone an unfair competitive advantage, and doesn't violate any terms of service.