First off, while Doubled Die coins produced from Dies with Doubling on them is technically an Error, they are considered as Varieties because they Error occurred in the production of the Dies. There are quite a few Error coins that can be and are considered to be one of a kind because of the Date, Mint, Denomination and Series and no others show for the same but somewhat similar Error coins of different Date, Mint, Denomination and Series may show up. One of a kinds are generally unique as they are rare errors. The ones that come to mind are struck Split Planchet coins, coins with multiple Strikes, flip over Double Struck coins, wrong Planchet strikes but there are others.