[bug] archiving creates duplicate entries

[Samuel Wales]

when i do a bulk archive, it will sometimes put 2 copies
entries into the archive file.  it is more likely to occur
if i am archiving more tasks.  at times, it seems certain
entries do this repeatably, but i am not sure.

there are times when i think i have reset everything (git is
clean, archives are moved out of teh way) but this still
occurs.  i cannot make an mwe but it seemed worth reporting.
where is it getting the duplicates from?  idk.  it has to be
either the source .org file or the newly created and written
archive file.  idk if there are any caches or text
properties or some hidden stuff in the agenda.  but i can
tell you that there have been times when i experimented with
not cleaning everything and the duplicates increased.  each
run would create a new duplicate of certain entries.

btw i have been trying to archive tasks to files for a year
now.  it is too slow for me.  so i hit on the idea of moving
archives out of the way, then archiving a little at a time
to new archive files, then using the shell to append the new
archived entries to the old archive files, then moving the
old archive files back.  thus, this isn't an issue of big
archive files.

wish i could provide more for you or even figure out
debugging but i cannot; just hope it will ring bells.