I think it is well-understood that Star Trek has almost never handled time travel with much consistency to speak of. Any time someone travels to the past, the effects of their actions on the future are handled in any number of ways.
However, as far as I can recall, the doppelganger issue is the one thing that is typically not messed with. Either the traveler goes to a time and/or place where they don't have to worry about their doppelganger, or they do have to worry about them and either avoid or interact with them as deemed necessary by the plot.
Then we have the Enterprise episode Shockwave.
In this episode, Archer is transported 10 months into the past - back when he was on Earth, just prior to the events of the series premier. However, instead of being brought back as himself from the current time, he seems to have been transported into the body of his former self.
This is perhaps better (or worse, I'm not sure) explained in the exchange between Archer and Daniels.
ARCHER: So, you're telling me you brought me back, what, ten months ago? How about Jonathan Archer ten months ago? Where's he?
DANIELS: He's you
Is this form of time travel, thus far unique to the Star Trek universe, ever explained? It did seem slightly reminiscent of Bakula's Quantum Leap days - I was actually a little disappointed at the lack of an "Oh, boy!". But I've never seen it in Star Trek before.