Some users complain about repeated requests for user name and password whenever they accessed the site which is very irritating. With some users entering the user name and password a few times it worked, and some users would request 10 times and then show the site, and sometimes would just result in a blank, empty page. While entering username password so many times, some users accidentally enter wrong password and end up locking their account. When I faced the issue, I tried to switch the browser, and voila it worked.
If you search the issue, you will find a lot of references to authentication providers, registry issues, trusted sites or intranet zone settings or maybe the users are on the server and the loopback check was not disabled.
But why it sometimes worked and sometimes it didn't.
I found that to some extent Content Advisor, Web sense, Firewall ,Proxy settings, either of it could also be a problem.
Turns users had a proxy server, with proxy settings deployed to desktops via group policy.The group policy forces the browser to ask for authentication on some content which according to the policy may be inappropriate. The proxy would behave differently every time it was asked for the new web site address, and would cause an authentication prompt for each image on the page
You could confirm this yourself by temporarily switching to a different browser than your default one. If the new browser doesn't take the policies or Content Advisor settings your default browser and your site works - you found your answer.You can ask network admins to either add the web site to the proxy exception list or fix the proxy server itself or switch over to a new browser or revise the Content Advisor.
Note: This article doesnot consider the repeated authentication prompts on Web Server due to loopback issue and Alternate Access Mappings. You may refer
Guide to Alternate Access Mappings in Sharepoint
