[GHC] #9056: --make paths are not deduplicated

[GHC]

#9056: --make paths are not deduplicated
-------------------------+-------------------------------------------------
       Reporter:         |             Owner:
  evincarofautumn        |            Status:  new
           Type:  bug    |         Milestone:
       Priority:         |           Version:  7.8.2
  normal                 |  Operating System:  Unknown/Multiple
      Component:         |   Type of failure:  Incorrect warning at
  Compiler               |  compile-time
       Keywords:         |         Test Case:
   Architecture:         |          Blocking:
  Unknown/Multiple       |
     Difficulty:         |
  Unknown                |
     Blocked By:         |
Related Tickets:         |
-------------------------+-------------------------------------------------
 I had a build script with an accidental duplicate dependency, which caused
 GHC to be run like so:

 {{{
 ghc --make Main.hs Main.hs
 }}}

 This results in a spurious error from `GhcMake.checkDuplicates`:

 {{{
 <no location info>:
   module ‘main:Main’ is defined in multiple files: Main.hs Main.hs
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9056>
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