Often yes, but that breaks easily. Referers are often not sent nowadays. Screen size and OS are often similar. Browsers now auto-update, so the versions are increasingly the same, along with plugins, which are getting rarer. Port numbers vary by connection, so you can't match them. IP address don't work well for people behind NAT, which is particularly common for mobile connections, which often have carrier-grade NAT[1], and for workplace/university/school networks.
Say you have two users on Macbook Airs using auto-updating Chrome (which has bundled Flash) in the same office or university, and both came in through Google.
Say you have two users on Macbook Airs using auto-updating Chrome (which has bundled Flash) in the same office or university, and both came in through Google.
Is this an unlikely scenario? Not really.
[1] https://en.wikipedia.org/wiki/Carrier-grade_NAT