Pulp

The migration for modulemd data introduces a new collection in the DB but doesn't create indexes before migrating a content.
So it's possible to have duplicated modulemd units in the DB.
Users would run into a problem during upgrade because pulp-manage-db will try to build missing indexes.

E11000 duplicate key error collection: pulp_database.units_modulemd index: name_1_stream_1_version_1_context_1_arch_1 dup key: { : "django", : "1.6", : 20180307130104, : "c2c572ec", : "noarch" }
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pulp/server/db/manage.py", line 236, in main
    return _auto_manage_db(options)
  File "/usr/lib/python2.7/site-packages/pulp/server/db/manage.py", line 303, in _auto_manage_db
    migrate_database(options)
  File "/usr/lib/python2.7/site-packages/pulp/server/db/manage.py", line 148, in migrate_database
    ensure_database_indexes()
  File "/usr/lib/python2.7/site-packages/pulp/server/db/manage.py", line 186, in ensure_database_indexes
    model_class.ensure_indexes()
  File "/usr/lib/python2.7/site-packages/mongoengine/document.py", line 738, in ensure_indexes
    collection = cls._get_collection()
  File "/usr/lib/python2.7/site-packages/mongoengine/document.py", line 210, in _get_collection
    cls.ensure_indexes()
  File "/usr/lib/python2.7/site-packages/mongoengine/document.py", line 766, in ensure_indexes
    collection.create_index(fields, background=background, **opts)
  File "/usr/lib64/python2.7/site-packages/pymongo/collection.py", line 1380, in create_index
    self.__create_index(keys, kwargs)
  File "/usr/lib64/python2.7/site-packages/pymongo/collection.py", line 1290, in __create_index
    sock_info, cmd, read_preference=ReadPreference.PRIMARY)
  File "/usr/lib64/python2.7/site-packages/pymongo/collection.py", line 205, in _command
    read_concern=read_concern)
  File "/usr/lib64/python2.7/site-packages/pymongo/pool.py", line 211, in command
    read_concern)
  File "/usr/lib64/python2.7/site-packages/pymongo/network.py", line 100, in command
    helpers._check_command_response(response_doc, msg, allowable_errors)
  File "/usr/lib64/python2.7/site-packages/pymongo/helpers.py", line 189, in _check_command_response
    raise DuplicateKeyError(errmsg, code, response)
DuplicateKeyError: E11000 duplicate key error collection: pulp_database.units_modulemd index: name_1_stream_1_version_1_context_1_arch_1 dup key: { : "django", : "1.6", : 20180307130104, : "c2c572ec", : "noarch" }

The migration should build indexes before migrating content.
The migration should handle duplicates properly.
See example of how a similar case was handled here.