PostgreSQL: Pgbouncer authentication

We are using authentication file mode in pgbouncer. The authentication part of Pgbouncer is so confusing. Would be helpful if anybody can throw some light on my below queries. - Are Pgbouncer connections **authenticated twice** at bouncer side and DB layer? - What's the use of authentication at Pgbouncer if it is using the **same credentials as database**? - Is it possible to use a **different set of user credentials** for the Pgbouncer authentication other than DB credentials itself?