When trying to add a new PostgreSQL server at Tools & Settings > Database Servers and specifying the option Host name or IP address as localhost and leaving the Port number field blank, the following error is shown after clicking OK:
PLESK_ERROR: Error: A database server with this combination of host address and port number is already registered.
PostgreSQL is installed on a server with Plesk using instructions from the PostgreSQL official wiki.
Record about PostgreSQL server with the hostname
and port 5432 already exists in the Plesk system database
This is a Plesk bug with ID PPPM-6955, which is planned to be fixed in future Plesk updates.
Until a fix becomes available, add a new PostgreSQL server specifying 127.0.0.1 as a Host name or IP address.