Block duplicate requests from FixedColumns

This is code written by @lwyszomi , which I have isolated and PR'd separately from the original custom reports PR: https://github.com/dimagi/commcare-hq/pull/3701

@biyeun @snopoke 
I'm putting this on staging.
@millerdev @esoergel  @dannyroberts This in not needed. I forgot to remove, sorry.