As a security measure, Fever is only meant to be used by a single user per device (mobile or tablet). This means that each device should only be used to access one profile.
If a friend or relative accesses their profile on your device, neither of you will be able to redeem vouchers. It's also possibly to accidently access the App using different accounts of Facebook or Google, creating two different profiles.
If this happens, you should delete all of the Fever profiles associated with your device except for the main account you want to keep. You can remove accounts by accessing the profile you want to delete, and entering "Account settings" where you have the option "Delete account". (Remember that by doing so, all your data, including any purchases that you might have made with this account, will be lost).