I have had 3 sets of twins and they have all been identical. Even the same likes/dislikes which I find silly. So I cannot answer the question if there are un-identical twins possible but I doubt it. Just because they give identical twins identical personalities.
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"Things to remember. . ." "First, you forget names. Then, you forget faces. Later, you forget to zip up your fly after exiting the men's room. Finally, you forget to zip it down."