The Dalek Duplicates were
Humanoids genetically engineered to serve the Daleks. These lifeforms were duplicates of soldiers killed in battle by the Daleks, and were used as undercover agents and troopers.
Although conditioned to obey their masters, duplicates retained some of the memories and thought patterns of their original selves, leading to a struggle against Dalek mental conditioning. This led the duplicates to become unstable, scuppering any Dalek plans to use them to infiltrate other planets and prepare them for conquest. (DW: Resurrection of the Daleks)