Posts can be accessed through multiple url in multilingual sites

I have a site running an instance of EasyDisccuss 3 (with lots of hacks to support multilingual) and am trying to upgrade it to EasyDiscuss 4. Here is another issue with the version 4:
Because you don't have a language field in post table so the post can be accessed via multiple url in multilingual sites. I.e: site.com/en/post-title and site.com/vi/post-title both give the same content and it is really bad for SEO.
Please fix it as soon as possible. Tks!