Getting trolled on a forum and...?

[Agent]

It's extremely easy to work out whether a user has created multiple accounts, as they can check which IP address created and accessed the accounts. An IP address is a special string of numbers and sometimes letters (in the case of IPv6) that uniquely identifies your computer online.

All they have to do (I've done this many times before to check for duplicate accounts) is get the IP address from your first account and see if it's the same as the IP address from the second account. Forum software (the most common being IP.Board and Xenforo) usually have a tool to do this built into the system.