When batch rendering you can't export multiple versions of the same file, say to render frame 1-87 in one movie and then export frame 233-420 in another. The second file gets the same name as the first already rendered and simply overwrites this.
Adding incremental numbering for batch rendering would be a good solution, maybe as an option or if there's already a file with that name the new file gets and incremental number added to the file name.
::EDIT::
Sorry, this was meant for "Feature requests", please feel free to move it LM.
Incremental file numbering in Batch export
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