How to ensure that the acceptance letter received is real and not a mistake?
The best way to know that you are not in a mix-up is to email a real person. As an international student you really need to speak to someone and you can ask for it. I recommend you to set up an skype interview, even before they accept you. I am sure you have tons of questions besides if you are accepted or not, and you should worry more about finding an education that is a good match for you. Still, I wouldn't assume the worst, that mix-up was an isolated case and not the norm.