In a site collection I have defined a Content Type called Person at the top level of the site collection.
In a sub site within the site collection I have added a list. On the list settings for this list I added the Content Type. Data has subsequently been added to the list.
Then I have saved this sub site as a template and created multiple other sub sites using this template. When the sub sites are initially created there are no duplicate content types with in the list.
Now I have noticed that there is a duplicate Content Type appearing in the List Settings. It has the exact same GUID as the content type that already is added to the list. Neither of the Content Types can be removed from the list because data already exists within the list.
This should not be possible because the content type has already been added to the list but it has happened.
The duplicate site content types on the lists do not add duplicate site columns to the lists.