[GHC] #15994: Duplicate entries in -ddump-minimal-imports

[GHC]

#15994: Duplicate entries in -ddump-minimal-imports
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider
 {{{
 module Foo where

 import System.IO

 f = [ReadMode, ReadMode]
 }}}
 We get this:
 {{{
 $ ghc -c -ddump-minimal-imports Foo.hs
 $ cat Foo.imports
 import System.IO ( IOMode(ReadMode, ReadMode) )
 }}}
 Notice that `ReadMode` appears twice.

 Turns out this is because of a refactoring in `RnNames`
 {{{
 commit 6353efc7694ba8ec86c091918e02595662169ae2
 Author: David Eichmann <EichmannD at gmail.com>
 Date:   Thu Nov 22 14:48:05 2018 -0500

     Fix unused-import warnings

     This patch fixes a fairly long-standing bug (dating back to 2015) in
     RdrName.bestImport, namely
 }}}
 (which I wrote -- don't blame David!).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15994>
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