login with SAML creates NEW user

Hi,

I am using moodle 2.5.1/ apache 2.0 / php 5.3.10

This is the situation. I have a database with users enrolled in different courses. They login by going to our site and log in. Now I want these same user to be able to login through another organization.  I installed the latest SAML authentication plugin (also simplesamplphp).

So when an existing user now logs via this organization he is redirected into our moodle, but then the next thing happens. If it is the first time, he has to enter his first name, last name etc, and then moodle creates a new user. 

This is NOT what should happen. The existing user should see what he normally sees when he login directly to our moodle. How can I solve this?