Nationwide car company has refunded me twice.?

HI

I recently purchased a car from a well known car company. I also purchased an extended warranty. After reading about the warranty I realised it wasn't worth the paper it was written on and used my right to cancel. Warranty was cancelled and I was sent a refund by cheque. The same day as I received the cheque I had already sent the company a letter requesting the refund. A week or so later I have received another cheque from a different department as a refund.

Now I know I should rip up the cheque and send it back. But am I breaking the law if I bank it and see what happens?? If they realise there mistake I would obviously reimburse the money without question and just act dumb!!