nothing. they're just giving you twice as many points per £1, so you can get a bit more back in the next mailing in vouchers..no saying when it ends, but it implies its to give you more before Christmas, ie. when you need a bit more help with money.
Glad I've not booked my holiday for next year yet, using clubcard/Virgin vouchers, so will definitely wait for a bit longer to get all my double bubble ones!