In his research for a way to pacify the massive undead population swarming the globe, Dr. Sam Isaacs created a multitude of clones of Alice. Using their blood, Isaacs devised a serum that was supposed to increase the intelligence of zombie test subjects. However, while the serum did succeed in temporarily increasing the zombie's intelligence, it actually increased their ferocity and the strength of the infection. Three such clones are featured in the storyline of Resident Evil: Extinction.
The first clone seen in the film. She woke up in the bathtub, just as Alice did years before, wandering through a replica of the Spencer Mansion and The Hive. She survived the laser room, and made it all the way to the end of the test grid, a recreation of the hallways of Raccoon City hospital, where she was killed by an automated Machinegun. Her body was dumped onto the surface.
Activated midway through the film, this clone also makes it all the way to the end of the testing grounds, only to be ambushed and killed by an Enhanced Zombie.
Woken up during the fight between the original Alice and the mutated Dr. Isaacs, this clone activated the lasers in the testing ground, using them to slice Isaacs to pieces. This clone is present with Alice, watching as the other clones begin to stir.
Dozens of dead clones are also seen in a disposal ditch on the surface, near the entrance to the underground lab. Going with information presented in the film, there should be 87 dead clones in all, since Clone #86 was the first activated in the movie, and died, along with Clone #87.