I'm noticing duplicate notes in the database, which might happen when
the ingester and writer get spammed with the same note rapidly. Add a
sanity check during the write so that we only ever write a note once.
Fixes: 1cf898e0b2 ("ndb: update nostrdb")
Changelog-Fixed: Fix duplicate notes getting written to nostrdb
Signed-off-by: William Casarin <jb55@jb55.com>