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Download Airpass for Mac
Wifi networks collect a reference to your computer (called MAC address) when you login. That way, when you try to login again, even with different credentials, they detect that it is the same device and do not allow you to continue using it.
By renewing your device's MAC address, the wifi network will not identify your computer and therefore is tricked into thinking it's a new device. When you try to login again it won't trigger the error message and you're free to use it.