Duplicate iCal Entries after Outlook sync

I am having a lot of trouble with iCal.

I have Microsoft Office for Mac 2011 and ipad2. 

I have a lot of calendar entries in Outlook which I like to see them in iPad. After turning on the sync services, I found that there are duplicate entries in iCal.   The duplicate entries only exist for those "all-day event" but no others.

As I have used Calendar since 1998 so there are probably hundred of those duplicates over the time. I just bite the bullet by deleting duplicate entries in the hard way. week by week and entry by entry. Unfortunately, iCal is taking like 30 seconds to delete two or three entries. So this is not working for me as I do not like to spend days in deleting all those duplicate entries.

I delete all iCal entries, clear the plist file, cache, etc and re-sync again. And duplicate entries come again.  

Are there ways to avoid duplicate entries after Outlook sync?

Please help.

Many thanks,

Samuel