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 kind 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.