Normally, if a payment is made by legitimate mistake, there is a fundamental right to recover the money. But, if the person receiving the money can show that they changed their position detrimentally in reliance on the belief that they were entitled to that money, they have a partial defense and are entitled to keep the money but only to the extent their position changes. An example of this would be if you were denied unemployment benefits because of the extra funds and had to use this money in order to sustain your lifestyle (An Australian example can be found here).
That said - wages are a payment for time worked. You have done nothing to earn this money and ethically, and very likely legally speaking you are expected to pay the money back. If this were to go as far as a civil suit you are going to have to try very hard to defend your position. This is going to cost you time and more than likely money and reputation - is it really worth it?
You need to accept that they made a mistake in paying you the money and you made a mistake in not questioning the extra funds in your account. I would either pay them back immediately or follow Joe's advice above and arrange a payment schedule.