To prevent fraud we need to ensure as much we can that customers don't create multiple accounts from their systems to access freebies and to do fraud. What we need is unique device id like Smartphones have and we can access that via android/ios applications, but in case of web/mobile browsers we can't fetch their device id/ serial numbers etc and somehow asking customers to allow us to fetch their system's information doesn't seems to be a good idea.
What we did initially was we md5 encoded customer's ip address and their user agent, this solved purpose by a tiny bit that we were able to identify multiple accounts being run from same ip and same browser, but still IPs can be changed very easily, Is there any way we can create something unique for each browsers linked with a system.