Star Trek: The Janus Gate — Future Imperfect
{4.5/5} “The shock of hearing that he was already dead on this planet, in this time, had hit him with unexpected force. It wasn’t dismay or disbelief that made Sulu’s breath catch in his suddenly dry throat. It was the horror of realizing that the immense explosion he had been flung into was his own funeral pyre, that he had actually witnessed the moment of his other self’s death.”
Star Trek: The Janus Gate — Future Imperfect by L. A. Graf, published in 2002
On a rescue mission on Tlaoli 4, Chekov finds his captain — sort of. James T. Kirk has gone through an alien machine and come out 20 years younger. He’s now a teenager. When Chekov tells him he’s from the Enterprise, Kirk says he knows that Robert April is the captain.
Elsewhere on Tlaoli 4, Uhura finds Sulu. But it’s a Captain Sulu from 20 years in the future. He has been at war with the Gorn, and has seen both Uhura and McCoy die. He has never heard of James T. Kirk.
This is the sequel to Present Tense.
It’s about dealing with people who are not from your time.
I’ve read 9 books by Graf.