I had the same issue...but I had several phones syncing multiple ways...
Started with desktop outlook-->synced to my eris--> synced to my laptop, which had my gmail sync program on it
the desk top also had the sync program on it, but it was defaulted with my girlfriends gmail so that her gmail and phone synced with my desktop..and then I could sync my phone with desktop or laptop, and then gmail and all calendars would be together...
We I ended up with 3 events for each event...
So I wiped all calendars out, fixed the desktop outlook calendar, synced my phone with it, and put the gmail synce on for defaulting to my girlfriends gmail, and occaisionally mine...and havent had any duplicates since. seems to be working now for a couple weeks.