second domain on same multisite redirects to wp-signup.php?new=

I have a multisite with domain mapping setup for one of the subsites and we added a second domain for the main site The second domain is parked so we can get email for that domain.
I tried multidomain plugin and still did the same thing. when typing in http://seconddomain.org it goes to http://maindomain.org/wp-signup.php?new=seconddomain.org
Both domains go to the same dns. main domain IP is shared.