In cases of extra limbs or duplicated facial features - always an absorbed twin?

When a baby vertebrate is born and they either have extra arms and/or legs, or there’s duplication of facial features (sometimes a whole face) is this 100% due to absorbing a twin in the womb, or is there any possibility that you might get an animal or human baby with an extra limb or second face that’s entirely their own? Can our own DNA “blueprint” get things wrong and start creating extra parts in the womb?

Double-tailed goldfish.

Duplication of facial features (Diprosopus) can also be caused by mutations causing excessive activation of the sonic hedgehog homolog gene
Wikipedia link