[GHC] #9939: Warn for duplicate superclass constraints

[GHC]

#9939: Warn for duplicate superclass constraints
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        Reporter:  crockeea          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"c790fe87be648f420c63099934852013a4e8a8f7/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c790fe87be648f420c63099934852013a4e8a8f7"
 Test Trac #9939
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9939#comment:3>
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