[Performance] why do subdirectory and subdomain work simultaneously

[Hovatek]

I created a forum subdomain.site.com and it works fine. I noticed that site.com/subdomain also works. this is bad for seo. what can be done to make only subdomain version work

[.m.]

you can use redirect rule in .htaccess file (assuming that your forum is on Apache server)

Redirect 301 /subdomain/ http://subdomain.site.com

[Hovatek]

you can use redirect rule in .htaccess file (assuming that your forum is on Apache server)

Redirect 301 /subdomain/ http://subdomain.site.com

thanks.
how do I enforce the search engine friendly url format?
such that if someone visits subdomain.site.com/showthread.php?tid=x , it'll be automatically redirected to subdomain.site.com/thread-x.html
I already have the search engine friendly url setup. just dont want duplicate content issues