[GHC] #13392: ghc --show-options lists flags twice

[GHC]

#13392: ghc --show-options lists flags twice
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        Reporter:  ThreeFx       |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  low           |            Milestone:  ⊥
       Component:  Compiler      |              Version:  8.0.1
      Resolution:                |             Keywords:  newcomer
Operating System:  Linux         |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by SantiM):

 Hello,

 I'd be happy to take this issue as my first contribution to GHC. I wanted
 to ask a few questions first though:

 1) I didn't find tests on the 'ghc' command or the "showOptions"
 function[0]. Any suggestions on where to put the test for this change and
 how to go about it?

 2) We could fix this by just adding 'nub' to the showOptions, but I guess
 that is not good enough and we probably want to take a look at how flags
 are registered in DynFlags. Ideas?

 Thanks

 [0] https://github.com/ghc/ghc/search?utf8=%E2%9C%93&q=showOptions&type=

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13392#comment:1>
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