Was in my local Tesco(Watford extra) yesterday and saw the offer of 4 pack buy one get one free on the duracell AA size.
Picked up four packets which worked out to be 16 batteries @ £5.96. While walking around the toys section saw the 8 pack Duracell which had 4 + 4 free written on them at the same price of £2.98 so I got rid of the 4 pack and changed them for the 2 x8 pack batteries.
When I got home and check the reciept it had BOGOF on the bottom for the duracell batteries with -£2.98. Which means I have 16 batteries for the price of only £2.98.