Clone

A clone is (usually) a biological copy of another biological entity. Clones can be produced to be exact replicas with short lifespans, as the Fourth Doctor did for himself and Leela at the Bi-Al Foundation. (DW: The Invisible Enemy) Or they can be clones of individual cells which are grown up into a useable organism, as Cassandra O'Brien did for herself following the destruction of her 'front' on Platform One. (DW: New Earth)

The Sontarans are also a clone race, making each Sontaran almost identical to the other, however the Sontarans have altered their biology during their long war with the Rutans to make them more formidable in battle. (PDA: The Infinity Doctors)They also used Martha to make a clone who could infiltrate The Doctor's plans by using her memories.