Clone

Information
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 the skin taken from the front of her body, on Platform One. (DW: New Earth)

Uses of Cloning
A clone of Martha Jones was made by the Sontarans to obtain an insider of United Nations Intelligence Taskforce. The clone has a mental link with the real Martha Jones and can therefore access her memories, the clone cannot live if this link is broken. They used the clone to try and stop the Doctor interfering with the conversion of the Earth's Atmosphere and their invasion. (DW: The Sontaran Stratagem/The Poison Sky)

Using blood samples from the Sixth Doctor, The Forge operative known as Nimrod created a multitude of clones in order to discover the means of time lord regeneration. All these clones where of the Doctor's sixth incarnation which suggests that time lords blood changes completely during regeneration, none of these clones can regenerate, they only have a 10% chance of surviving birth and do not live very long anyway. Nimrod has since abandoned this project (BFA: Project Lazarus)

Races which use Cloning
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).