Here is the scenario. User changes his/her username in Active Directory. This causes some of the site collections to have both old and new user names. InfoPath forms and Workflows use the old usernames and hence cause the forms or workflows to fail. In order to resolve the duplicate profiles, try the following PowerShell command.
Move-SPUser -Identity DOMAIN\old_username -NewAlias DOMAIN\new_username
Wait for a full user profile service to SharePoint sync to complete. The duplicate profile should be gone now.
Sometimes the above PowerShell command throws an error and to be executed in a different way. This is explained in this post.