The episerverframework.config file has built-in support for this.
See http://world.episerver.com/Blogs/Eric-Pettersson/Dates/2011/6/Use-to-use-se-and-com-adresses-without-an-enterprise-license-EPiServerFrameworkconfig-to-the-rescue/ for an explanation,
and http://www.epinova.no/blog/Thomas-Leela/dates/2012/12/managing-episerver-languages-and-site-hosts-made-easy/ for a plugin to handle this better.
I have a website with two languages enabled, x1 and x2, x1 one is default (this is necessary by architecture configuration). But now I can access the same page in two ways: www.abc.com/testpage and www.abc.com/x2/testpage. Which is the best way to make a permanent redirect from www.abc.com/x2/testpage to www.abc.com/testpage, as I need to have only one URL for the page. Looks like I have several options like HttModule or some URL Rewrite rule. Is there any built in way to do that? Which is the best option?