I was thinking the other day about trackers and their ability to detect duplicate accounts. I would think they compare ips, cookies and maybe mac ips (if this information is available when you log in to a site). Are there other more sophisticated techniques?

I'm just wondering because I was thinking if I were to open my own tracker, what sorts of things I would need to think about.

I missed an obvious one which would be checking email addresses. I also just thought about checking common MD5 strings? Potentially if someone used the same password it might at least flag that someone might have a dupe account. I welcome other thoughts.

You cannot see MAC addresses.

Is there other information that is available to identify a specific device then?

If you want the best solution go and ask what.cd staff.

There were some veeeeeery long discussions on reddit whether what.cd can get info about MAC addresses. It was never proven that they can but to be fair it was never dissproven either.

It is not possible from a technological point of view. MAC addresses are only used in LAN. Your ISP knows it, the Wi-Fi network you're connecting to etc.