please answer!

if i was accidently paid twice by my previous employer is it illeagal to keep that money?

Yes legally you do need to repay if you agree it is an error; you can however if true argue hardship in having to pay all or any substantial portion--saying that it was their mistake after all, and you spent it w/o realizing it was in error, and paying back now would be difficult...they often will not sue or take any further court action in these cases but will try to work something out. They may even settle on a partial amount.