Was very easy actually not to notice it, god hate it when people on this site just give smart answers not funny and not helpful, and it thats the only bit of power you get in your lives its sad. I asked a question didnt ask for a comedian
Well Durrrr if you want to stay employed and on good terms you have to pay it back ! It will look bad already in how long you have left it ! A guy in my work was leaving and they paid him 6k extra by mistake ! He immediately noticed it and informed HR who were extremely grateful, he gave the money back and they gave him 100 euro one for all vouchers!
Your employer is entitled to deduct it- where there is a clause in your contract saying this and also under statute where the monies have been paid as a mistake