It's a criminal offence (under Section 5(4) of the Theft Act 1968) not to return property (including money) which you have acquired through the mistake of another, where there's an obligation to do so (which there clearly is here). So, yes, you've got to return it.
Yes, they can add it to next year's bill, but they'll probably treat it as a separate bill. Unlike most unpaid bills (which are purely civil matters), failure to pay Council Tax can lead to criminal proceedings. (i.e. there are two different criminal laws which you could fall foul of if you don't repay the money).
You're situation isn't uncommon. I've been waiting for a Council Tax bill of around £30 for several months (covering a period when I was temporarily 'signed off' JSA, and not eligible for council Tax Benefit, which I normally receive in full). However I recently received a 'refund' of nearly £150 instead of the bill I was expecting. I keeping quiet about it for now, but I expect that they'll demand it back again eventually!
Chris