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SFO sync fails with - An item with the same key has already been added error message

Knowledge Article Number 000187940
Description ISSUE

Sync fails with an error  like this

Unable to sync. Review the appropriate log for details.

A sync error occurred. Review the appropriate log for details.

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Unable to sync. Review the appropriate log for details.

An item with the same key has already been added

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You can also check the Trace.log located at %AppData%\\Salesforce for Outlook\ and you will see this error
Resolution This issue could be related to either a duplicated Redemption key in the registry or some corrupted records in Outlook.

To resolve this issue, you can search the registry for a duplicated key and if it does not resolve the issue you can check Outlook Contacts, Calendar, or Tasks folder (depending on what object the sync fails on)  for any events that could cause the sync to fail. This is a trial and error method and you need to check the SFO Sync.log or Trace.log files to see what records could cause the error to pop up. If you are syncing from Salesforce to Outlook and you create all your records in Salesforce, try to sync into a brand new Outlook folder such as a new Contact or Calendar folder and see if the sync works. If it does then create a back up folder and move all your records from your default Outlook Contact or Calendar folder and sync again to bring down all the records again from your Salesforce into your Outlook

1- Close Outlook and SFO

2- Click on Start and type in Regedit

If you have Windows XP click on Start | Run and type in Regedit

3- In Windows Registry Editor, navigate to


4- Make sure you have only 1 entry starting with 5
For example, as of SFO 3.2.5 you should have the 5.4 entry in this location

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If there are any other entries such as 5.2 or 5.5, export them by right clicking on each and click on Export and save it on your Desktop and then right click on and delete them.
Next reinstall SFO. Do not delete the 4.5 entry

5- Open SFO and Outlook and try to sync

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