Example from The Wotch: Anne Onymous accidentally schedules 4 different things for the same time, and uses a spell to multiply so she doesn't have to cancel anything. Unfortunately, it turns out that the duplicates are actually aspects of her personality, and when the spell overcharges, all the aspects of her personality run loose. Cue showdown against her personified Anger, who later becomes a recurring antagonist...
Dr. Positron in PS238. There are several Dr Positron androids, with slightly different personalities.
In Homestuck, Aradia Megido's time powers in the Medium allow her to create an army of alternate future selves, which are used to help defeat the Black King in their session. Dave Strider has the same powers in the kids' session and (ab)uses them to the point where there are at least 43 iterations of him running around at one point, both Stable Time Loop travellers and Bad Future copies. Also, Eggs from the intermission has the ability to teleport into the past, which he uses to create an army of... hims; he also creates an army of Biscuitses too by taking Biscuits with him when he teleports. In the Alpha Session, Dirk Strider has several versions of himself — his real self, his dreamself, his auto-responder which is now part of his sprite, and the "splinter" of himself lodged in Jake's subconscious. It's apparently part of his abilities as a Hero of Heart. Having so many versions of himself around is actively causing him to begin to hate himself.
Since the arrival of the Prospitian warship in the Alpha session, Jane and Nanna are currently the only Sburb Players to have both of their instances (post-scratch player and pre-scratch ancestor/guardian, respectively) alive in the same universe simultaneously. Beta Jack Noir, Spades Slick and Alpha Jack Noir are all currently together in the Alpha session as well.
Schlock Mercenary turns this up to the max. Gav stepped through a duplicator device with 950 million outputs, and the clones now form a major sapient ethnicity and marketing demographic. Cloning Blues eventually gets to Gav, and they start funding and implementing a program to physically and mentally distinguish themselves.