Problem
During a SharePoint migration, you noticed in the logs the following error message:
“Error migrating content type [id]: [Name of the element]: A duplicate content type [Name of the element] was found”
Cause
Content types are kinds of metadata sometimes used by developers to customize some documents or views.
These are probably system metadata already present in the target (provided by the template used to create the target site).
Resolution
You can simply ignore these error messages.