Content Deployment was failing with the following error in the logs:
Title: Field name already exists. The name used for this field is already used by another field in the list. Select another name and try again.
Object: User Information List
We had a publishing site that was upgraded from SharePoint 2007 to SharePoint 2010 that was using content deployent from a staging site to a deployed site.
We could see using SharePoint Manager 2010 that we had duplicates in the destination site that were not in the source site.
We opened a job with Microsoft and we found someone with a similar issue on the msdn forums
Microsoft gave us an executable ‘RemoveDuplicateColumn64‘ to remove the duplicates, we ran it for each duplicate.
The exe prompts you for all the required parameters, one of which is the duplicate internal name which in our case was FirstName0, again we used SharePoint Manager to find the internal name.
We have no idea how these duplicates got there but since removing them a couple of months ago there have been no problems with the content deployment.
This problem could likely be avoided by using the new snapshot content deployment available in SharePoint 2010.