-- First, I recommand getting the size of the database to copy, as this might be important for the rest of the process.
SELECT pg_database.datname,pg_size_pretty(pg_database_size(pg_database.datname)) AS size
FROM pg_database;

-- Create the new database. This might take some time depending on the size of the database.
CREATE DATABASE my_new_database TEMPLATE my_old_database;

-- Then, change the owner of the newly created database.
ALTER DATABASE my_new_database OWNER TO new_dbuser;

SELECT pg_terminate_backend(pg_stat_activity.pid)
FROM pg_stat_activity
WHERE pg_stat_activity.datname = 'originaldb'
AND pid <> pg_backend_pid();