Another student has been assigned the same MSc thesis as mine (and already defended)
The answer is very simple: ask your current supervisor and be completely transparent with them about the situation. Whatever is acceptable to them is the right answer. Standards for masters theses are much more flexible and inconsistent than for doctoral theses, so, for all practical intents and purposes, whatever your supervisor approves is acceptable. (But this assumes that you are absolutely transparent and do not withhold any information about the situation from them.) 

But be sure to email a recapitulation of their response to them and let them reply, confirming that you have understood their resolution correctly. That way, you have an official written record with dates so that there is no forgetfulness or confusion in the future. (In fact, that's a general tip for any important verbal conversation throughout your professional life.)