I have some kind of web application - you pay for account and then you can use services it provides. Now I would like to detect if two or more users are using one account - and than warn user or disable this account.
I really don't know where should I start, so can you tell me anything ? Is there already completed solution for this problem (maybe open source) ?
I could compare IP addresses, but that could result in false-positives (maybe user is accessing my web application from his mobile phone and few minutes later from his computer) ...
You can do something very simple - when you login from one location, you automatically log out all other locations. This wont completely solve it, but it will make it more annoying for 2 or more people to share an account.