We tried to inherit from IPublication behavior as a replacement for the default behavior and put a new field on the dates tab. Now if we put the fieldset in there two Dates tabs are showing up (IPublication is disabled).
Yes. Its disabled.
If you dont believe it: Clone the git-repo local, buildout, create plone with dx types and add the behavior TTW to a portal-type of your choice and remove the default IPublication.
Then add the type and look at the tabs.