Duplicate calendars

Hi, I'm syncing my calendar via ActiveSync on my iPad and iPhone and for some reason, I'm seeing duplicate calendars listed on both devices. Incoming event invitations are similarly duplicated - and annoyingly, refuse to remain as "accepted", so that they constantly reappear in the invitations queue no matter how many times I try to accept, although rejecting any instance will remove the event from "both" calendars. I've tested and verified this behaviour from my own Gmail account, any assistance would be greatly appreciated.