Unable to create a content type – A duplicate content type was found

 
I found my problem, so if this can help some people some day. Looks like SharePoint have some problems sometimes to update a field in the content database. The field NextChildByte from the dbo.ContentTypes table is the problem.

Lets say I create a content type with ID 0x010100ACEA2663B318874AA9192CA9AF678614 with Document as parent (0x0101). SharePoint will create an entry in the dbo.ContentTypes table.

When I create an other content type with the first one as parent.. The ID of this new content type will be 0x010100ACEA2663B318874AA9192CA9AF678614*01*) This 01 came from the NextChildByte field from the dbo. ContentTypes table. Now SharePoint should update the dbo.ContentTypes table and set the field NextChildByte to 2 so the next content type created using the same parent will be 0x010100ACEA2663B318874AA9192CA9AF678614*02*

My problem was.. SharePoint didnt update the NextChildByte field so SharePoint was trying to create a content type with ID 0x010100ACEA2663B318874AA9192CA9AF678614*01* when it already existed…
 
In my case.. I finally have been able to delete the content type 0x010100ACEA2663B318874AA9192CA9AF678614*01*. When I recreated it, SharePoint did update the NextChildByte to 2 so I was able to create a more child content type after that.