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Most of the vouchers offering massive discounts from well known brands are usually scams. If you want to check whether they are genuine or not, I would suggest go to the brand's official Facebook page and see if they have posted that offer on their page or not. Ditto for competitions, do not enter any personal information, unless the brand is running the competition from their official Facebook page. Lot of people have been fooled by McDonald, Tesco, Morrison vouchers.
There are few genuine vouchers too. Just double-check their authenticity before you click. Although do remember, most of the times if it's offering a huge discount and it looks like it's too good to be true - it probably is.