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Shared login over multiple domains isn't possible, simply because the cookie set for one domain cannot be accessed through another domain (it's just how the web works). The best option, that is also much better in terms of SEO is to pick one domain to host the site, and redirect all others to that domain. You can put this in your .htaccess 

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www.streetlegalmods.com$ [NC]
RewriteRule ^(.*)$ http://www.streetlegalmods.com/$1 [R=301]