The corresponding xapian document just gets more terms added to it,
but this doesn't seem to break anything. Values on the other hand get
overwritten, which is a bit annoying, but arguably it is not worse to
take the values (from, subject, date) from the last file indexed
rather than the first.
There are many other problems that could be tested, but these ones we
have some hope of fixing because it doesn't require UI changes, just
indexing changes.
