I happened across an old Deep Space 9 episode tonight where O’Brien keeps time-traveling 5 hours into the future. The freaky part is where he forces himself into the future to find out the cause of DS9’s impending destruction, dies, and his future self has to take his own place in the past. (That splitting sound you hear is my own skull being torn asunder.)
That being said, it made me wonder how often it is that somebody on any Trek show ends up getting duplicated for the audience. Some cases I can think of:
Picard: Ran into his duplicate from 6 hours in the future who fled from a destroyed Enterprise. The double faded out when his future was diverted.
Data: Has an evil twin brother, Lore, and another brother whose name I can’t remember from Nemesis. Lore was disassembled at the end of a two-part episode, with Data confiscating (and later using) his emotion chip.
Riker: Was accidentally split in two during a transporter accident. The “other” Riker took the name “Thomas.” Thomas joined the Maquis rebellion against the Cardassians and was apparently taken hostage (and eventually killed in a slave labor camp).
Crusher: No full duplicates here, but she once touched an icky goo that perfectly duplicated one of her hands.
Kirk: Was once split in two in a transporter accident. This wasn’t really a duplication so much as a splitting: one was good Kirk and the other was evil, and they had to be rejoined so that neither would die.
I can’t think of any others at the moment. For purposes of this discussion, though, Mirror Universe doubles don’t count. I mean, geeze, everybody theoretically has one of those.