I am facing  simillar (if not same) problem. I have One Portal site  which has two
additional Team sites. I added  a contributor to all three of these. The account
that I added is and AD account. Now, the user  keeps getting prompted on every
action (i.e. even while navigating) three times. Did you or any one find any way
on how to ensure the user logs in once and that's it???

Also, the user logs in on his/her computer with his/her same AD id, before
launching the IE browser and connenting to the site.