The original clone Edit
In 2004, the Forge created three clones of the Sixth Doctor using a blood sample gathered when Nimrod attempted to make the Doctor regenerate. The first clone died of hearts attack seconds after being born. The second lived for a few days until its throat was slit to test if it could regenerate. When the third began exhibiting Doctor-ish traits such as scientific reasoning and a sense of humour its life was spared. The Forge then kept it as their "pet Time Lord".
They made this clone of the Doctor their scientific advisor, call himself "the Doctor" and wear clothing similar to the Sixth Doctor's blue outfit. However, the original's DNA eventually "unravelled at the seams" and he died. His DNA was used to create another clone of the Sixth Doctor which had all the memories of the first clone. This one eventually died as well, and was replaced. The process continued for three years, with each new copy of the Doctor unaware that they were just one in a series.
The clone encountered by the Seventh Doctor Edit
The clone met the Doctor and pretended to be his previous incarnation, claiming that he had been forced to hide in the Forge after removing the TARDIS's Time Vector Generator to stop it being used by the Forge or the Huldron. He lost one of his arms when he was attacked by several Huldron, the Seventh Doctor realising that the clone was a fake both because he never remembered sustaining such an injury and because the damage didn't trigger a regeneration. After spending two hours recovering, he and the Seventh Doctor went into contact leading to the Seventh Doctor discovering the clone's true identity. The two Doctors then discovered that there were far more than three clones of the Sixth Doctor created. Enraged by this, the clone imitated Nimrod's voice and told Oracle to activate the Alpha Facility's self-destruct.