You need to read up on what constituted identifiable and personal data.
An email is not "Personal Data", if it were then websites and forums that ask for it wouldn't be in existence.
You can discriminate by browser, theirs 30 or more types of web browser in general use, these are on various devices like computers, mobile phones, PDA's and Tablets.
You could have two people at one IP address, one is a Linux build using Opera and another is a Windows Build using FireFox.
Each user at that IP address would be different and that would allow you by asking for an email address the ability to ensure that you are identifying two different browsers or connections.