Users IP Addresses

Hey,

I am working on a user login where I do not want more than one user logged in (from different computers).

The thing is, there are so many opportunities for an IP address to not be retrieved by PHP or the user could be behind a Proxy.

So, is there any way that I could do this?

Is the only solution to monitor how many logins and actions are taken by one user and if there is an unreasonable amount activity then make the judgement of whether there is more than one person logged in? (and basically not have a definite record)

Thanks!