Bounced check mysteriously cashed again - glitch?

I have multiple accounts with a large bank. I accidentally wrote a check, to pay a friend of mine, from a check book belonging to one of the old and now inactive accounts I have with this bank. My friend deposited it in his credit union account. After 2 weeks, his credit union sent him back a copy of this check stating a reject code of "Unable to locate account" My friend contacted me and told me that his credit union has charged him $15 for a bounced check. When he gave me the account number and check# for the check which I wrote him, I realized that it was not one of my active accounts with the bank. (Also the money was still sitting in my checking account) So I apologized and sent him another check via automatic bill payment. This amount was immediately deducted from my checking account and my friend got the money when he deposited this check. However after a week or so, my friend again contacted me saying that the first check (from the inactive account) I wrote him, has also been successfully honored and now he has twice the money in his account. I was clueless as I don't see any debit from any of my active accounts for the amount of the bounced check. My friend wrote me a check again to return the amount of bounced check. I have looked at all my active accounts but can't locate any debits in that amount.I have still not deposited the check my friend gave me, as I want to do more research going thru my accounts again. Is it possible that this happened because of automatic overdraft protection which my account has? Should I contact my bank? Will I or my friend be in any trouble if this happened because of a glitch in the credit union's systems which they uncover it shortly? My intention to write a check to my friend from a check book which is old (and inactive) were really legitimate and it was purely accidental. Since then I have torn apart that bad check book, so such errors don't happen in future. My question is whether I should deposit the check my friend gave me and let the bank and credit union figure out what happened OR should I contact my bank to update them about this situation? Can this be considered as bank fraud or something serious like that which can get me or my friend in trouble? If that's the case, I would like to know that before I deposit that check and cash it.