Postgres allows the use of any existing database on the server as a template when creating a new database. I'm not sure whether pgAdmin gives you the option on the create database dialog but you should be able to execute the following in a query window if it doesn't:

CREATE DATABASE newdb WITH TEMPLATE originaldb OWNER dbuser;

Still, you may get:

ERROR:  source database "originaldb" is being accessed by other users

To disconnect all other users from the database, you can use this query:

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

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