[GHC] #13697: Teach GHCi to deduplicate history

[GHC]

#13697: Teach GHCi to deduplicate history
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Here's a quick reproducer,

 1. Start `ghci`.
 2. Enter a command: `putStrLn "hello" <enter>`
 3. Enter another command; `putStrLn "hi again" <enter>`
 4. Press `<up arrow>` once to select the `hi again` command, press
 `<enter>`
 5. You will now need `<up arrow>` three times to get back to the `hello`
 command as the `hi again` command appears twice in history. This is in
 contrast to, e.g. `bash`, where it will only appear once.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13697#comment:3>
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