One-time cleanup for anyone already hit by the reimport-duplication bug:
groups existing rows by (source, raw_uid) -- backfilling a content-based
key first for rows that had none -- keeps the row a meeting is attached
to (or the first survivor otherwise), repoints any meeting pointing at a
row about to be removed, then deletes the rest. The unique index is
dropped and recreated around this since the backfill step can
momentarily produce rows that collide before they're deduped.
iamdoubz
e7b951cb90