Fix a Maniphest issue where multiple saved copies of a query break the menu

Summary: Fixes T2210. Recently, we require unique keys on menu items, but it's currently possible in Maniphest to save the same custom query under multiple names. Avoid exploding in this case (we'll hide the duplicates). This isn't a great fix, but makes Maniphest usable again.

Test Plan: Saved the same query twice, laoded page, got exception, applied patch, loaded page, saw duplicate query stripped.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2210

Differential Revision: https://secure.phabricator.com/D4247