Duplicate entries in -ddump-minimal-imports

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@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!).

Trac metadata

Trac field	Value
Version	8.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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