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

[iham]

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')