Q: Let's assume that during an interview, my interviewer gives me a case that I had already been given during my last attempt for the same company or while training with case partners. Should I tell him/her that I already know the case? Or should I keep it to myself?
I don’t think there is a realistic chance you will get the same case you received from that firm in the past. This would mean the company is using the same case they gave 1-2 years before AND you get the same interviewer. If you do get the same interviewer and the same case, you could mention you got the same case from him/her in the past, however I don’t think I ever heard of a situation like that.
On the other, I helped candidates who got the same case that was given to others (I have a 1.500+ questions DB and if the timeframe is short, I saw a candidate might get the same cases given). In that case, it is not possible for them to know that you know and it’s up to you to decide whether to reveal it or not.
Keep in mind that even if the prompt is the same, the case itself might be different later on (I heard stories of candidates knowing the case from another candidate, providing the solution to that case, which was wrong because the interviewer altered the case in the meantime). Additionally, there is a part of the evaluation which is not related to knowing the solution of the case.