I have a program that works with two devices (phone and laptop). On each device you can sign up and log in unless you have already done so. Once the user has either signed up or logged in on either device they cannot reach the log in/sign up page again and they just reach what they really want on start up.
The problem with this, is that if the user, for example; First signs up on the mobile device. And then following that, signs up on the laptop (instead of logging in) they will be permanently stuck without the two programs working in conjunction... Unless the client re-downloads either program (never what one wants)
What should I do to prevent this from happening?