While the duplication of Pokemon can totally lead to a weak spot in Pokemon trading (ie someone selling 5 shinies for 300 each that they cloned), it isn't a bad thing since they are technically legit and extremely hard to obtain. I think if you do decide to clone, we should have a rule that only one selling of that exact pokemon is allowed to be made from the user to avoid cheap PC making tactics. If they have multiple but are different IVs/natures and such (meaning they aren't cloned), then it won't be a problem. I personally clone any of my special pokemon also in case something happens to my game I can get them off my younger sibling's game to transfer over if I have to buy a new game or so.