So the feature is not intuitive at all, but exist.
You have to copy the classe from the Model explorer, and copy it in the Model explorer under the model you want it. Now you have a copy of your class in the Model explorer, and you can drag and drop it in your diagram, and voila, you have a clone of you class.
But yes, a clone functionality directly from the diagram would be nice.
So the feature is not intuitive at all, but exist.
You have to copy the classe from the Model explorer, and copy it in the Model explorer under the model you want it. Now you have a copy of your class in the Model explorer, and you can drag and drop it in your diagram, and voila, you have a clone of you class.
But yes, a clone functionality directly from the diagram would be nice.