Force SSL / username combination

Hi,

I want to set up a database server that is connected to the Internet.
Therefore, I want my users to use SSL/TLS certificates. I provide these
certificates to the users, so I can control the information that is in
there. Now, I know that I can set up PostgreSQL to use SSL/TLS, but I am
unable to find a way to map a SSL/TLS certificate to a specific user.

Suppose that user A and user B both have a different, but valid
certificate. They can both use their certificate to connect to the
server, but then use the username of the other user to log in. I want to
make sure that each user is connected with his/her specific certificate.

Is there a way to set this up? If so, how?

Thanks for any pointers!

Koen