Nestene duplicates were plastic duplicates of humans created by the Nestene Consciousness.
Characteristics
You have only seen the Autons. Crude weapons with a single offensive function. The facsimiles like Scobie's are perfect reproductions, even down to brain cells and memory traces.
In addition to the basic Auton, more agile and maneuverable models were made as duplicates of humans. The Swarm Leader was able to create the body of a planet's native life-form from scratch; (PROSE: Synthespians™) the first Auton created, Channing's human appearance allowed him to infiltrate Auto Plastics and organise the Nestene invasion. (TV: Spearhead from Space)
More sophisticated Autons could be created to copy and impersonate specific individuals. The originals needed to be kept alive to maintain the copy, although these were sometimes imperfect, looking and acting human except for a slight yellowing sheen or glint to the skin and a flat-sounding voice. When destroyed, the duplicates reverted to the appearance of a basic Auton, sometimes leaking Nestene blood. (TV: Spearhead from Space) One Auton copied Graham Winslet before reverting back to its basic form; however, it later appeared as Winslet with no apparent connection to the real man, although it may have only required a human appearance rather than Winslet's mind. (HOMEVID: Auton, Auton 2: Sentinel, Auton 3: Awakening) Occasionally if a copy wasn't necessary and an Auton simply needed to appear human, it could wear a plastic mask. (TV: Terror of the Autons)
Even more accurate Autons could be made, identical in appearance and actions to the originals. These units were implanted with false memories and actually believed themselves to be the real subjects they duplicated, being indistinguishable. The Auton copy of Rory Williams was even able to remember being erased from existence in the time field. (TV: The Pandorica Opens)
A number of famous figures were duplicated and displayed at Madame Tussauds, including General Scobie whose duplicate was intended to take his place and command the British Army. (TV: Spearhead from Space) Mickey Smith was copied to gather information on the Ninth Doctor. (TV: Rose)
Roman Auton duplicates taken from a memory print of Amy Pond mimicked humans perfectly and were used as sleeper agents until the Nestenes later asserted their influence, although the Rory Auton partially resisted this control. (TV: The Pandorica Opens)
History
The Tremas Master made use of his own duplicate. (GAME: Destiny of the Doctors, HOMEVID: Destiny of the Doctors)
River Song claimed to have dated a Nestene duplicate with a swappable head once. In regards to the head, she quipped that it "[kept] things fresh". (TV: The Big Bang)
When the Nestene Consciousness joined the Alliance (TV: The Pandorica Opens) following its 2005 invasion of Earth (PROSE: Revenge of the Nestene) it duplicates of Roman soldiers in 102 (TV: The Pandorica Opens) as part of the Total Collapse Event Incident. (PROSE: Dalek Combat Training Manual) These, believing themselves to be genuine Roman soldiers, tricked the Eleventh Doctor, who realised too late what they were. They imprisoned him in the Pandorica. One of these Autons, fashioned after the late and time-erased Rory Williams using the memories of Amy Pond, did not want to fire on her and tried to resist the call to arms. However he was unable to prevent himself shooting her fatally. (TV: The Pandorica Opens)
Afterwards, the Auton Rory helped the Doctor resurrect Amy with the Pandorica and save the universe. After the Doctor reset the universe and time, the Alliance Autons were erased from existence. The real Rory Williams, though, recalled the life of his Auton copy. (TV: The Big Bang) However, Justin Richards, an alientologist of the 21st century (PROSE: The Secret Lives of Monsters) in the restored universe, (TV: The Big Bang) uncovered evidence of Autons being active at Stonehenge and found documents talking about the Legends of the Lone Centurion, even including a picture of Rory in his book The Secret Lives of Monsters in its section on the Nestene. (PROSE: The Secret Lives of Monsters)