Kyuande means that if you both use the same key from different locations (different external IPs) you get this error.
Either buy a second key or only play from the same house.
But beware when you do the latter.
Although multiclienting is not illegal, it is frowned upon by the community and will usually result in a ban from the server.
So if you do it, it is better to only do it in your own server or on servers where you know the host.
It is better to just wait for a Steam sale and get an other key for like 3$.