I noticed that the item Metal Cloak is only available once per game/save state and that means you can only evolve a single Scizor or Steelix. Now are there some other items that occur only once per game and that don't require you to steal it from a wild Pokemon to get a second one?
The items you mean are Evolution-inducing held items. These are often only avalible once in a game. Here is a list: Evolution-inducing held items
Also there are story and basic items you only get once like a bike and so on.
Which Pokémon evolve while holding an item you can see here: