[GHC] #12389: Limit duplicate export warnings for datatypes

[GHC]

#12389: Limit duplicate export warnings for datatypes
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11959            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by parsonsmatt):

 I'm going to attempt a patch for this. By my first look, it should require
 a change to the `compiler/parser/Parser.y` Happy file, specifically the
 `qcnames` parse rule.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12389#comment:12>
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