Accidentally cashed a check twice

The bank would most likely notify you via mail to tell you that you cashed a check on mobile deposit and then cashed a check at the ATM. The letter should show your balance or the the two deposits with one cancelled.

This is check fraud, be careful, and don't run back to the bank without planning (but plan fast)

That is likely not as bad as it sounds. People occasionally bounce checks (also check fraud), and a single slip-up will likely be forgiven if you are proactive.

Before going to the bank and explaining the situation

Make sure you actually did mobile deposit the check (1st time) then re-deposit it in person (2nd time). Most banks will have check photos available online. Print this out and bring it to the bank. They should have an image of the 2nd deposit on file. Don't go to the bank without the image of the first check. Banks are required to maintain records for 5 years

Don't go to the bank without a check or a repayment plan

If you can just withdraw the extra money and write a check back to the bank that's the best plan. It's unlikely any bank would do anything but take the money back. Remember to get the amount in writing from the first bank and take a photo of the check before you deposit it back. If you cannot do this, go in with a repayment plan.

Keep all the records on file. If there is an issue after-the-fact, you want to be in a position to prove you fixed the mistake.

NOTE: Check was cashed at a "Check-2-Cash" place, not a bank

This will not work in your favor. These places are shady, which makes it look more like an intentional deception on your part. You still need to go in with a plan (see above), but this means you should move twice as fast to make sure this gets resolved.

And whatever you do, DON'T USE ANY PLACE WITH A NEON SIGN TO CASH A CHECK! Use a bank!

This type of situation is all about initiative. If you contact them first, it looks like an honest error you caught. If they discover it and contact you first, then it looks like you tried to get away with something and they caught you.

Of course, in the latter case, you will protest that it was an honest mistake, but that just sounds like you're saying that because you don't have any other options now that you got caught. So you will have zero credibility.

So Right Now do whatever it takes to get caught on tape trying to tell them ASAP. If they are closed and don't have voice mail, I would write a note, put it in an envelope, and slide it under their door, making sure to get caught on CCTV doing so. That will put a timestamp on your efforts.

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Unless this was a tax refund loan (which is often offered routinely to people who e-"file" with certain companies), your statements do have a credibility issue. One gets a loan for a reason, eagerly awaits the check, and applies the money to the reason. It's rather improbable to "forget". That, plus cashing it once with your bank and the second time through a check cashing place, raises some suspicion about your motives. All the more reason to take the initiative and get recorded with the earliest possible timestamp, trying to honestly rectify the situation.

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Only as a standard disclaimer, I mention those online scams where "employers" "pay" you to cash checks into your personal bank accounts and send money on to them. We have many Q&A on that, read up if it applies to you.

The best method would be to contact the check cashing place, and have them stop processing. Whether or not that's possible depends on the actual company you used. Talking to the company that you got the loan from will also probably work, and will usually involve you giving the full value of the check back. (That's including whatever the check cashing place took as a fee, so you will be out some money this way.)

This is a common mistake, and as long as you aren't intentionally double-depositing to steal money, you'll just have to give the money back. There's no real need to panic over this, just fix it.