What's the Name of That Book??? discussion

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Liminal States
SOLVED: Adult Fiction
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SOLVED. Adult Scifi book I read around the early 2000s about a pool in a cave that creates a copy of a person and destroys the original. Spoilers ahead. [s]
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The book covers several hundred years I believe and there is eventually a woman who he shows the pool too or she discovers it herself. I remember some tension at some points where one or the other would be in the pool and the other person was waiting for them to come out to kill them, because if you die outside the pool you still die normally: you have to go back each time.
I think maybe it always resets your new body to the age you were when you first entered it, like it remembers you and all the other people who ever went into it from that first entry.
SPOILERS BELOW!!!
I think the climax is they discover the pool comes from a huge alien grub like thing and someone else has created a huge sort of factory to collect the stuff, but the creature is slowly dying and producing less. They decide to sneak in and destroy it to let the creature free.