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hg export (Mercurial > Export Diff) seems to append the diff to the output file. So if you export and export and export, the output file grows and grows and grows. It should be overwritten, IMO.
Well, not so easy to fix, we can not delete the file before the actual 'hg export' since we don't know the exact name of the output file, there may be several wildcards in the filename. Let's deal with this when someone notices.