Hooks running twice...anyone seen this before?

I was having some weird issues with some add-ons hitting errors, and after some testing, I found that the sessions_end hook is being run twice every time the CP loads (same for session_start). I have a feeling this has something to do with one of the addons, but I dont really know where to start looking. Has anyone ever seen this before or know a good place to start?

thanks!

Bryant