Do not spend it, the employer has three years to request it back. https://www.ecclesia.blog/zu-viel-gehalt-gezahlt-welche-moeglichkeiten-hat-ein-arbeitgeber#:~:text=Eine%20Rückforderung%20ist%20demnach%20innerhalb,von%20drei%20bis%20sechs%20Monaten.
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It got sent to you by mistake so they have several years to ask for it back. So contact them and tell them.
Big companies are big, small companies often outsource their accounting. In both cases it can take quite a while for the news that you quit to reach the person responsible for the payment.
As others said: contact them and be prepared to pay it back.
Give it to me!
I'd just call them or look at your contract. Usually you can't quit and stop working immediately. You have a period where you have to keep working and of course you would still be paid for that period. But just talking directly with them should be the quickest and easiest way to find out what's going on.
Is there a risk they can ask for it back?
Yeah, they have 3 years starting from the end of the current year. Plenty of time.