Thanks for the replies. Here's a crappy pic of the problem.

https://animation.filmtv.ucla.edu/stu...ena/subdiv.gif

Edit: Ok, it turns out there was something called polyToSubdShape1HistPoly was parented to polyToSubd1 in the Outliner. Deleting that took care of the problem. I still have no idea why the vertices doubled in number when converting to a poly. Sorry if my terminology is unclear, I'm still learning exactly what everything's called.