Experience in is less than experience out, and so it duplicates experience at rate close to 2:1 but not quite. My setup to test this is simply an XP drain, a tank and an XP shower.

Edit: as I say this, I tested Kibe on its own and it must be another mod causing it. For posterity, I'll leave this issue up and report back when I figure out which mod causes it.

Turns out it was Overpowered Mending. Does some funky stuff with experience orbs, I guess. I'll post an issue over there. Sorry for the inconvenience!