Multiple simultaneous user access with same credentials

I have a ASP.NET website where multiple users access the site using same user credentials. The site functions fine when used by single user, however when a second user with the same credentials logs in simultaneously and the first user performs any action (clicks any link, selects any dropdown etc.), the first user is redirected to the default/login page. I want both the users to access the site without any trouble.
Can any one comment on why this is happening and how this can be solved.