If you synchronize files that Unison insists to differ but that are in fact
already identical, synchronization fails claiming that "Destination updated
during synchronization".  There is no apparent way to make Unison accept the
fact that the files indeed match except for nuking all of Unison's data and
setting up the replicas anew.

Unison uses not always true assumptions to avoid checksumming, and thus it
can err in some cases.  Thus, it needs to accept that rsyncing may yield no
differences.