I've been out hunting slaves and ran into something odd, not sure if it has been reported before or if it is a known issue.
In one trip I rescued and recruited the same person twice, like exactly the same. So much the same that after selling both to the market and click on either of them in the list of slaves for sale they both get highlighted. Looking through the list of slaves in the shop some more I see that it has happened at least once before as I have 2 copies of another person in there.
My best guess is that when you recruit them in the reward screen for rescuing a captive they aren't getting removed from the lists of people in the world. Which is kind of annoying, I am recruiting them to remove them from the pool and I don't want to ruin my reputation by killing them, or capturing them which would also make them less valuable unless I take the time to calm them down afterwards.