--make paths are not deduplicated

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

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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