We are using authentication file mode in pgbouncer. It's so confusing the authentication part of Pgbouncer. Would be helpful if anybody can throw some lights on my below queries. 

Does Pgbouncer connections **authenticated twice** at bouncer side and DB layer?  

What's the use of authentication at Pgbouncer if it using **same credentials as database**?

Is it possible to use **different set of user credentials** for the Pgbouncer authentication other than DB credentials itself?