seraphime authored Jul 01, 2016 and Ben Gamari committed Jul 04, 2016

Fixes issue T12165 by banning duplicate pattern synonyms
signatures. This seems to me the best solution because:

   1) it is coherent with the way we treat other duplicate signatures

   2) the typechecker currently doesn't try to apply a second
       signature to a pattern to see if it matches, probably because
       it assumes there is no more than one signature per object.

Test Plan: ./validate

Reviewers: goldfire, austin, mpickering, bgamari

Reviewed By: mpickering, bgamari

Subscribers: mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2361

GHC Trac Issues: #12165