Why would anyone do this? We get several of these threads each year around this time of the cycle. If you didn't want to go there, then why apply? And if you realized after your interview "I can never see myself waste four years of my life here" then you should have withdrawn post-interview.
They may or may not see an acceptance, but some applications ask if you have been accepted before. They will know if you matriculated, so if you've sent in all your forms to start classes you're in an interesting situation.
Be glad you got an acceptance and not one of the thousands across the nation holding hope onto a waitlist or the even more who only have rejection letters sent to them. Take the acceptance and run.
Ah, then maybe it was secondaries. I haven't gone back and looked, so you can likely ignore me. However, I do remember several threads on this topic and it seemed to be a big deal that you'd have to explain in interviews. "Why did you turn down X acceptance?" In which case, I imagine somewhere you have to say that you turned down an acceptance, otherwise how would they know?
Some secondaries will ask if you've ever been accepted, rather than matriculated. Also, after March 15th (IIRC), schools can see through AMCAS where you have been accepted. It wouldn't be a stretch to say that a school could look at your old application and see you turned down an acceptance, and have you explain why that is the case if you make it to the interview stage. Maybe an adcom member can clarify if they can look at an old application and still receive the AMCAS traffic data.
I reapplied after being accepted. Yes, they will generally ask you about it at your interviews. Some schools have it on their secondary, so I assume that is how they know. FWIW, I did not receive an interview this year at any school that interviewed me last year, so maybe some schools won't interview applicants two years in a row.