Transmogrify from 5.1.5 to 5.2 Edit page fails

I used a transmogrifier pipeline to upgrade from 5.1.5 to 5.2.0 and when I try to edit a page I receive this stack trace:
Traceback (innermost last):
Module ZPublisher.WSGIPublisher, line 155, in transaction_pubevents
Module ZPublisher.WSGIPublisher, line 337, in publish_module
Module ZPublisher.WSGIPublisher, line 255, in publish
Module ZPublisher.mapply, line 85, in mapply
Module ZPublisher.WSGIPublisher, line 61, in call_object
Module plone.z3cform.layout, line 63, in call
Module plone.z3cform.layout, line 47, in update
Module plone.dexterity.browser.edit, line 58, in update
Module plone.z3cform.fieldsets.extensible, line 65, in update
Module plone.z3cform.patch, line 30, in GroupForm_update
Module, line 141, in update
Module, line 52, in update
Module, line 48, in updateWidgets
Module z3c.form.field, line 277, in update
Module, line 443, in update
Module z3c.form.browser.text, line 36, in update
Module z3c.form.browser.widget, line 171, in update
Module Products.CMFPlone.patches.z3c_form, line 47, in _wrapped
Module z3c.form.widget, line 132, in update
Module, line 176, in toWidgetValue
Module, line 399, in get_vocabulary
Module, line 235, in call
Module zope.schema.vocabulary, line 128, in init
ValueError: term values must be unique: u'r-beer'

Where r-beer is my username. My pipeline is pretty vanilla only substituting:
# Here's where the actual magic happens, create the content object in the site
blueprint = collective.transmogrifier.sections.constructor


For some reason, the vocabulary being created seems to see the value "r-beer" a second time. Check the code in near line 235. Find out from where it gets its values and why the value above can occur twice.

If necessary, I would debug the affected function in an interactive Python interpreter (--> "bin/[instance|client1] debug`).