WP accepts up to 60 international cadets at a time. Candidates apply through the American Embassy in their respective country. According to the WP Class Profile online, there are 15 international cadets currently attending. There's more info on the USMA website for international cadets. From what I remember reading somewhere, they attend and participate in the identical 4-yr program as US cadets. At the end of four years, they commission into the branch/service of their respective country's military.
Perhaps what you saw was the semester exchange cadets from the Royal Military College of Canada. I don't know how many are currently at WP, but my DS said there was one in his company last semester. (He hopes to do an exchange at RMC at some point in his 4 yrs.) Last weekend was also when many cadets from RMC visited WP for several days, participating in a hockey game and other competitions. WP took home the Challenge Cup along with the chess, crew and debate titles. The Pipes & Drums groups and Celtic dancers from both WP and RMC put on a show before the hockey game.
Also, you can not have dual citizenship at a SA. You have to renounce the foreign citizenship. I'm pretty sure the question ("do you have dual citizenship?") is somewhere in the appointment paperwork (or maybe it was in the initial application?).