My concern is about SEO. Now let me explain the scenario.
I am making a 3 languages website. The development is alright, but I was targeting local customers with one domain, and international (English version) with another. Eg:
http://www.minhalojadesapatos.com.br (this is not the real website, just example!)
Both domain point to exactly the same hosting and content, but when user comes through local domain, default language is set to Portuguese, otherwise, default is English. Language handling on backend uses PHP Sessions and cookies, so with just a click users can change content language.
How to avoid being SEO-penalised in this context? (yeah, I was hungry when focusing market for choosing two domains but the activity really needs that, it is a travel agency).
If client enter by domain #1, he/she will see a Portuguese page. But there is a link on top to change language for English (this link contains a rel="nofollow"). Clicking this link, a session variable will be set, and content will display English, which contains the same content as domain #2 directly accessed, and vice-versa. That's my concern about duplicate content.