Do you avoid linking users to the sign in page directly?
When you are adding a hyperlink which links to a web application that requires a login, do not use the login page (login.asp or login.aspx or login.php) for the value of the "href" attribute, use the default page (or other pages) instead.
Thus, if a user is already logged in, he will go to the default page. If not, the page will redirect him to the login page. But if you use the sign in page, the user has to sign in again though he's already logged in.