I plan on battling my friend soon with an Eviolite Porygon2 and Eviolite Chansey. I want to destroy said friend, but I don't want to do so if I have an inherent advantage. I've heard that more than two of the same items is okay, but I'm now sure.
It depends whether or not your friend follows the item clause.
The item clause states that two Pokemon on a team cannot hold the same item. However, alot of people don't bother with the item clause. The only way to know whether or not your friend considers it cheating is to ask him.
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It depends on your established rules. Competitive battling on the OU/UU/Uber scale allows for repeated items, while the official Pokemon online rules do not. If you use 2 Eviolites, nothing is stopping your opponent from using a team full of Choice Band users.
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Nintendo uses the Item Clause more than Smogon does.
So.... So I technically could use an Assault Vest team, as long as I don't do any....official battles?
Nintendo's official rated/tournament formats enforce the item clause. No one else does, definitely not Smogon (not their standard rules, anyway). So yes, go for it.
As for TC, I say go for it. Using two of the same item doesn't really doesn't make much of a difference in power, though you may want agree on it beforehand or not do it if your friend is the type who will take any excuse to avoid admitting they lost fair and square.