All of a sudden, this morning, Apple Mail started downloading emails to my gmail account despite the fact that I knew there were no new emails. I looked at the inbox and discovered that there were four copies of each of the recent email. I cancelled the sync, and Mail promptly started to sync twice as many emails. After a few attempts I had approximately 32 times my inbox preparing to sync, so I closed Mail and left this way.
Does anyone know what happened, why, and, most importantly, how to fix it?
EDIT: After restarting Mail the problem has gone away. However, I still want to know what happened and how to stop it from happening again.