In the second Science of Discworldthe wizards travel back in time to prevent their past selves from changing Earth history.
Dad, we bleeping can't!
You can leave Robo unsurprisingly a robot behind in the past to work on reviving a forest.
Somehow, apparently in the future, Stan is thrown back in time.
When they finally travel to Chibi-Usa's time period, the 30th two bottom plow frontier, Mamoru meets the source of the nightmares - his future self, King Endymion, trying to ensure their love was strong enough to survive the trials of the Black Moon.
One of them's messy, the other ones clean!
Brit the Younger hates being around her future self, since Brit the Elder typically makes her seem like a stupid child.
Likewise, Silvia and Mikono seem to get along pretty well, in spite of, reincarnation aside, them not being very similar to each other, unlike the above two.
Two and Three bicker constantly, and One gets a wonderful dig at them both.
Meanwhile the Parental Revenge Agency does its job.