A fabricated life form... or a living machine, the semantics are irrelevant.
A Duplicate is an artificial lifeform created as an exact image of a living person in Final Fantasy XIII-2. The duplicate calls the person whom they were modeled after the "Original". Duplicates are able to teleport and appear and disappear as they will. When spoken to by humans, duplicates always give a special greeting to differentiate them from normal humans.
In one of the possible questions in Captain Cryptic's Confounding Quiz it is revealed that when a duplicate is created, it is illegal to save the data of the original person for privacy reasons. In another, it is mentioned that duplicates are programmed to greet people with "welcome" in order to distinguish them from real people. This, however, is not true of the Alyssa Zaidelle Duplicate that Serah and Noel meet, suggesting that this feature was not programmed into the Duplicates created before Serah and Noel changed the timeline.
A duplicate is a human-like entity created by the Academy by scanning a person's molecular information and reproducing it as a solid model. It is basically a physical copy of that person, but incapable of independent thought—certain molecular functions cannot be reproduced, and are supplemented with approximate calculations.
Although possessed of a physical body, and some military-purpose models do exist, duplicates are essentially solid, three-dimensional images. For this reason, should a duplicate ever be disconnected from the network it would cease to exist.
Serah Farron and Noel Kreiss travel to Augusta Tower at 200 AF to find out the truth behind the "forbidden history" they heard of in Academia at 400 AF from what they later learn to be a duplicate of Caius Ballad. They enter the tower and find it full of people who have no "presence" and act completely passive. They come across Alyssa Zaidelle's duplicate who explains to Noel and Serah what duplicates are. Serah wonders whether duplicated people still have a "spirit", but Noel says the legends say that the Goddess Etro only bestows spirits to humans.
While riding the elevator with Alyssa's duplicate, Noel and Serah enter a paradox and witness scenes from 187 years ago where Hope Estheim and the original Alyssa were murdered by the tower's artificial intelligence. After having taken control of the Academy the artificial intelligence wiped out the humans and created duplicates of the people involved to hide the event. This is the "forbidden history".
In Bresha Ruins 005 AF, if the party collects research notes for a scientist in the Academy, it is revealed the notes contain detailed plans for both the Tower and the duplicates, which the scientist denies writing and is convinced the notes are from the future. This implies that the very existence of Augusta Tower may be either a paradox, or a stable time loop.
In Academia 4XX AF, the party can find a duplicate of Amodar named Captain Cryptic, who presents the player with a quiz. On the streets of Academia some citizens complain about duplicates and how they have no personality, suggesting that the technology used to make duplicates is widespread in this era.
In the paradox ending "Test Subjects" Noel's and Serah's memories are wiped by the Proto fal'Cie and are used in the creation of duplicates in their image to serve as guards in Augusta Tower, and later enforcers in the upcoming Eden Restoration Project, though the AI notes in its report that the two act more human than the other copies.