XF 1.4 non-www appears as duplicate site

1) I have a wordpress/forum setup where the forum is a subfolder i.e www.mysite.com/forum.
2) On my blog .htaccess file, I set up 301 redirect so whenever someone clicks mysite.com/forum they're automatically redirected to www.mysite.com/forum.
3) However, when I check my referral traffic (in wordpress, for example) I occasionally see some clicks from mysite.com/forum (non-www). When I click that, my forum is seen as a completely duplicate site, with me not being logged in at all.
4)I should note this happens only with the forum part of the site, not the blog part.
5)Google Analytics whines that I have redundant host names (probably because of the above).
6)Below is my forum .htaccess file (from the /forum subfolder). How should I fix this?

# Mod_security can interfere with uploading of content such as attachments. If you
# cannot attach files, remove the "#" from the lines below.
#<IfModule mod_security.c>
# SecFilterEngine Off
# SecFilterScanPOST Off

ErrorDocument 401 default
ErrorDocument 403 default
ErrorDocument 404 default
ErrorDocument 405 default
ErrorDocument 406 default
ErrorDocument 500 default
ErrorDocument 501 default
ErrorDocument 503 default

<IfModule mod_rewrite.c>
RewriteEngine On

# If you are having problems with the rewrite rules, remove the "#" from the
# line that begins "RewriteBase" below. You will also have to change the path
# of the rewrite to reflect the path to your XenForo installation.
#RewriteBase /xenforo

# This line may be needed to enable WebDAV editing with PHP as a CGI.
#RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^(data/|js/|styles/|install/|favicon\.ico|crossdomain\.xml|robots\.txt) - [NC,L]
RewriteRule ^.*$ index.php [NC,L]