This bug shown up as part of a big merge, some files were conflicting, and some
other not. It occurred that I made exactly the same change than my colleague on
some files (but on a different branch). The changes are about some added lines.
(prototype function declaration in a C header file). When merging those two
branches, the magic occurs.

At the end, this file did not thrown the merge tool in the air, Mercurial
silently performed the merge without any complain, but the merged file has the
change twice. (twice the function declared in my header file).

I managed to reproduce it in the attached repo.

This repository has a couple of heads, my two first try were unsucessfull in
reproducing the bug (Thus the "sucess" commit message), but the two last one
show the bug. ("failed" commit message)

I categorize this bug as critical as it made me lose confidence in the merge
process.