Run both. The latter reports an error, however:
discourse=# REINDEX SCHEMA CONCURRENTLY public;
ERROR: could not create unique index "index_users_on_username_lower_ccnew"
DETAIL: Key (username_lower)=(marks) is duplicated.
I guess it’s time to get into PostgreSQL and to find and delete the record.
The vacuuming did the trick, more or less, but I still get peaks every few minutes with one to three postmaster processes consuming CPU for one or two minutes each. So I guess there’s more.