Raise exception if 2 document types have the same name

Summary:
To prevent accidents, this'll give a hard error if you try to override
something in the document registry (instead of just failing in a strange
way later).

Test Plan: Started server, fixed errors.

Reviewers: josh, yanwei, yiting

Maniphest Tasks: T5807

Differential Revision: http://infra1.corp.contextlogic.com/D9797