1) If you enroll at any school, you must withdraw your application from all schools that have given you a firm decision (ie. rejected/accepted). You do not need to withdraw your application if you haven't heard back yet from a school, have been waitlisted, or have been held.
2) So, if both X and Y have accepted you, and you enroll into X, you must withdraw from Y.
3) The only ethical way around this, I guess, is to approach X and be 100% honest and humble with them. I may also contact Y to see if they can be of any help in this matter. They may even offer you $ - you never know...