McDonald's customer service voucher stays valid

Uriziel

I received a customer service voucher for £10 from McDonald's and used it yesterday but it is still there and it allows me to use it again in a new order.
This is obviously an error and the terms of the voucher say one-time use.

Now I am wondering if I can just go ahead and keep using this? Can McDonald's hold me liable if they notice?

flaneurs_lobster

First Prize : Free McDonalds
Second Prize : .......