I sync my windows mobile phone to two computers.  It works fine, EXCEPT deleting a contact is next to impossible.  Example: create a contact, sync with each computer.  Delete it on any of the three devices, and it recreates it: the first sync removes it from both devices.  Upon syncing with third computer, it comes back.   I’m not sure how dupes are created, but I suspect it is similar: edit a contact in one place, and eventually it is seen as a new record, not a modified record.    Here is a small app to help:

http://msmvps.com/blogs/nunoluz/archive/2008/04/24/remove-duplicate-contacts.aspx