Each shopkicker is allowed to have one account.  If you have more than one device (if you have a smartphone and an iPad, for example), you can log into your account on both devices.  You'll get an informational notification (not an account warning) when you log in on a second device that you have done so (it can also show up if you've reinstalled the app lately, even if you only have one device) - this is just a reminder.  You can still do your regular shopkicking, just remember that you can only collect kicks on one device at a time, so you only need one when you go out shopping.  Also, remember, you may not share devices with other shopkickers, meaning no one else is allowed to log into their Shopkick account on any of your devices, at any time.