I suspect this means that you have a corrupt index. What version of Postgres are you running?
something like:
cd /var/discourse
cat shared/standalone/postgres_data/PG*
(I can’t quite remember the postgres filename).
You can search here for “postgres corrupt index” and find a topic that I once wrote about how to track down those bad records and delete them.
You basically try to rebuild that index and delete the records that it complains about then try to rebuild the index again until it rebuilds.