Ben Klopfer

In my experience, this problem exists in all version of CRM (4.9, 2011 and 2013).

When contacts are synchronized from Microsoft Dynamics CRM to Outlook, the Outlook plugin creates matching contact records in the user's Outlook account. Problems occur if a user re-installs Outlook, or migrates to an Outlook installation on a new workstation. Here’s where the duplicate contact records come from:

1. When the CRM for Outlook plugin synchronizes for the first time, it creates synced CRM contacts in Outlook
2. Outlook is installed on a new workstation or is uninstalled and re-installed on the same workstation
3. The new installation of Outlook sees the contact records, but does not know anything about the synchronization, so they turn into normal Outlook contact records
4. When the CRM for Outlook plugin synchronizes for the first time on the new installation, it creates new synced CRM contacts in Outlook.
5. CRM’s Duplicate Detection Rules only apply when synchronizing from Outlook to CRM and not from CRM to Outlook, so it assumes all the contacts are new
6. Now, Outlook has duplicate contact records for all originally synced CRM contacts: An Outlook copy and a CRM synced copy

Furthermore, if the duplicate Outlook contacts are themselves manually tagged or selected by filter to sync to CRM and there are not Duplicate Detection Rules in place, you can end up with duplicates in the CRM database also.