Client Identification Other Than Ip Address

Client Identification Other Than Ip Address

I run a php website where users can only have 1 account, this is a big
deal when users have more than 1 account. I use some IP checks to get an
idea of double accounts, but need to know if there is ANYTHING else I can
do. I know most sites don't care as it looks like more users, but this is
very important for this site that users only have 1 accounts! Getting Ip
addresses don't always work, just looking for any tips!