Amazing, miraculous human beings. They come from the splitting of a fertilised egg, and look exactly the same. They have the same DNA, so one could commit a crime and frame the other one. (Although this never happens because identical twins are loving, friendly, law-abiding people).
Do not confuse with Non-Twins
"Wow look at those two, they look exactly the same! They're helping that old lady cross the road, how kind!"
"Yeah they're identical twins, aren't they amazing?"