ValueError: ('Duplicate name', 'my.behavior')

created a package and registered a behavior "my.behavior".
works fine so far.

create another package (my.behavior.sub) and override the behavior.
(just needed to change languageindepent on one field of the behavior)

fire up zope: all is fine.
create a plone site and install additionally the my.behavior.sub
still - all is fine

but now go to control panel -> dexterity types -> choose a type and click on the "behaviors" tab:

ERROR Zope.SiteErrorLog 1489568742.040.915683235822 http://localhost:8050/Plone/dexterity-types/News%20Item/@@behaviors
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, in update
  Module z3c.form.form, line 226, in update
  Module plone.z3cform.patch, line 20, in BaseForm_update
  Module z3c.form.form, line 154, in update
  Module z3c.form.form, line 136, in updateWidgets
  Module z3c.form.field, line 227, in update
  Module plone.app.dexterity.browser.behaviors, line 115, in fields
  Module z3c.form.field, line 136, in __init__
ValueError: ('Duplicate name', 'my.behavior')

Reproduce the failure in the Dexterity type behaviors tab using the two packages and inspect plone.app.dexterity.browser.behaviors and the z3c.form.field call shown in the traceback. Determine how the overridden behavior is represented during field construction; done means the tab loads without a duplicate-name ValueError.