fix validation of multiple field having same db_field

Summary:
wasn't including field names from superclasses.

Adding a extra param dup_check to fields so we can ignore already broken fields
in model/notification

Logic:
1. two fields with different python name but same db_field --- BAD
2. two field with same python name and same db_field --- FINE
3. others --- FINE

another diff coming shortly that adds the dup_check param to broken
fields, please review them together

Blame:

Test Plan:
1. made sweeper use this version of mongoengine,
2. started server before adding the dup_check params, fails on notification
3. start adding dup_check params to broken notifications and watch the
failures go away, server starts after adding all of them

Reviewers: david, adam, josh

Reviewed By: david

Maniphest Tasks: T8023

Differential Revision: http://infra1.corp.contextlogic.com/D12403