I regularly sync my Windows Mobile phone with Outlook 2007 using ActiveSync/Vista Sync Center. I don't know what causes this, but every so often I end up with duplicate contacts and calendar items. This happens especially with recurring items like birthdays. If I am not quick enough to remove the duplicates, I end up with 2, 4 or even 8 of them!
In the past I have used the Duplicates Remover for Outlook from MAPILab for this. I used the trial edition that works for 30-days. It is limited to removing 10 items at a time, so I had to run it multiple times. I didn't use it often enough to shell out the $24/€19 to buy it.
Today I tried version 2.5.2 of this Outlook add-in. It has a wizard-like interface that is easy to use. I got my contact list and calendar cleaned up again.
[Full disclosure: By writing this review I am eligible for a free license for this Outlook add-in. This does not effect my opinion about this tool. Obviously, if I wouldn't find it useful, I wouldn't want to have a license.]