How to stop Duplicate contacts with Google Mobile sync for blackberry?

I am using Blackberry 9780 and configure Google mail. I installed Google Sync in my Blackberry for synchronize contacts and calender from Google mail. I have 2000 contacts and calenders in blackberry. my Google mail contacts and calender are empty. start Google sync software in my blackberry. Successfully sync with Google mail(all contacts and calender are updated in Google mail).
After one week i noticed some contacts and calenders are duplicated in my blackberry and also duplicated in Google mail contact. some contacts are 5 times duplicated. more than 200 contacts are duplicated.
weekly i am adding a new contacts in Google contacts and set Auto Google sync in blackberry.
now total contacts in blacberry = 2256(256 contacts are duplicated).
how to stop for create duplicate contacts? how its creating automatically?