Diagramming the results of consanguineous marriages is a special case. One that is not currently handled by this Report. You would probably need to file a Feature Request.
I created a test case (using the Example.gramps tree) by merging the spouses of 2nd cousins (Barry Joseph Garner & Barbara Jo Garner) into their respective cousins. (And trimming out some of the excess offspring to simplify the charts. Also, I turned off display of much of the rendering of detail to simplify the diagrams too.)
When charting the offspring in the Descendant Fan Chart, it looks like this:
Since Barry Joseph Garner is the cousin descended of the elder child in primogeniture, his offspring is are fully diagrammed in his arc of the fan chart. But he appears as a Greyed out segment in his wife’s arc… to indicate he is already diagrammed elsewhere.
The Descendant Tree addon Graphical Report completely omits a second listing of Barry as his cousin’s spouse. (That would be a feature request.) But there’s also a bug in that direct descendants are supposed to be Boldface font. It looks like it took the expedient of putting all spouses in plainface.
Meanwhile, the Family Tree graphical report addon shows the Duplicate offspring but has no indication that they are duplicated on the diagram.
Another proposed diagram has a more of a deep dive into diagramming inbreeding:
See the Contextual Family Tree in GEPS 030: New Visualization Techniques.