I am applying for residence next year as an upper year, and I am wondering whether I should try and hope for a single room or apply with a friend just in case I don't get a single and end up with a random roommate. However, if I apply with a friend, doesn't it diminish my chances of getting a single? In other words, am I automatically considered for a double?

Yes, it says on the application that a valid roommate request outweighs a single-room request.