Identical Twins

two identical individuals which develop from a single fertilized egg that divides early on into two equal parts. Identical twins have the same genetic make up and are always of the same sex.

Taken from Dictionary of Science

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Twins are two offspring resulting from the same pregnancy, usually born in close succession. They can be the same or different sex. Twins can either be monozygotic (MZ, colloquially "identical") or dizygotic (DZ, colloquially "fraternal" or "non-identical").

Taken from Wikipedia