this was driving me crazy and i searched here for solution and couldn't find. so i'm posting my solution so next time it happens i will find solution!

the calls to the script must use identical url, such as

1. http://domainname.com or
2. http://www.domainname.com.

if favorites link has #1 but script url is #2, will get second prompt to login. solution is to set favorites same as script url.