As seen is bugs/issue1014_identical_patches.sh
Reported by Ian.

This one bit me today.  The changelog comment should make it clear
what I was trying to do:

    code.haskell.org:~/public_html/ru$ darcs pull --match 'touch .bin/ru' ~/Preferences
    darcs: bug in get_extra commuting patches:
    First patch is:
    Sat Dec 27 00:38:53 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * Extract ru from larger repository.

      I didn't realize (until it was too late) that my ru patches depended
      on the patch that added .bin, which was a large "initial import"
      patch.  Cleverly, I discovered that under darcs-2 semantics, if I add
      .bin TWICE (in two separate patches), I can obliterate the latter
      patch without breaking ru's dependency on .bin/.

      This will still cause problems when pulling updates to ru from the
      larger repository, I think, but that's a damn sight better than
      permanently carrying around all those extra patches in this repo.
    Second patch is:
    Sun Nov 16 04:48:55 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * ru -- command-line interface to roundup.

The transcript below demonstrates a minimal procedure to produce this
behaviour by creating two patches that create the same directory, then
attempting to obliterate and re-pull one of them.

    $ with-temp-dir
    with-temp-dir: entering directory `/tmp/with-temp-dir.FXQCUW'
    This directory will be deleted when you exit.
    $ darcs init
    $ mkdir x
    $ darcs rec -lam x x
    Recording changes in "x":

    Finished recording patch 'x'
    $ touch x/x
    $ darcs rec -lam x/x x/x
    Recording changes in "x/x":

    Finished recording patch 'x/x'
    $ darcs init --repodir y
    $ cd y
    $ mkdir x
    $ darcs rec -lam y x
    Recording changes in "x":

    Finished recording patch 'y'
    $ darcs pull --all --summary --match 'touch x/x' ..
    Pulling from "/tmp/with-temp-dir.FXQCUW"...
    Will pull the following changes:
    Sat Dec 27 02:36:04 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * x

        A ./x/

    Sat Dec 27 02:36:17 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * x/x

        A ./x/x
    Finished pulling and applying.
    $ darcs cha --summary -p ^x$
    Sat Dec 27 02:36:04 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * x

    $ darcs obl -a -p ^x$
    Finished obliterating.
    $ darcs pull --all --summary --match 'touch x/x' ..
    Pulling from "/tmp/with-temp-dir.FXQCUW"...
    darcs: bug in get_extra commuting patches:
    First patch is:
    Sat Dec 27 02:36:55 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * y
    Second patch is:
    Sat Dec 27 02:36:17 EST 2008  Trent W. Buck <trentbuck@gmail.com>
      * x/x
    $

David's darcs has a fix for this:

  darcs get http://physics.oregonstate.edu/~roundyd/code/darcs

It'd be good if we could pick it apart and understand how it works. Asking David
would be a good idea as well.

Standard problem with duplicates. Unsolvable with current patch format.