In my opinion checking for duplicate IPs is an old school way of going about spam protection. There are better methods already integrated into XenForo. If checking for duplicate IPs was a sound way to protect your site, XenForo would have added it long ago, it's literally a couple lines of code.
I have a site running XenForo with over 175000 members using only the XenForo built in spam protection options and have zero issues that aren't caught by those methods.
I agree...using IP address to help block duplicate accounts probably is old school...but could be an extra layer to prevent duplicate accounts. I think the duplicate accounts activity usually comes from longtime members that have been inactive for a while...they've forgotten their login details...then they register again with a new account.
In the case of this thread...we only found out about the "duplicate account thing"...when a newly registered member contacted us with a request to change their account email address...and turns out that email address was already used in another account (this person from an old account).
But sometimes new members create an account...then don't like the username they created...then create a new account with a different username & different email address (instead of contacting us with a request to change their account username). In this case if the person is not using a VPN...they probably have the same IP address...and if the site was checking for duplicate accounts via IP address...this sort of thing would get flagged/caught.
We probably get about 20-25 new account registrations/day...the numbers can add up quickly.
As far as spam protection...the biggest issue is spam via the "Contact Us" form. We of course have CAPTCHA running...but this doesn't stop them all. Many folks say Q&A CAPTCHA is the best. But even with Q&A Captcha (the Q&A being site topic specific)...spammers still get thru. If the Q&A Captcha is too hard...then it can get too difficult for new folks joining...since they may not even know the answer (without looking it up). Lol
I'm going to guess (at the moment)...Captcha blocks about 80% of the spammers via the Contact Us form. I tried an experiment a while ago for about a week where Captcha was turned off...and spammers via the Contact Us for increased about 5x. After a week & Captcha was turned back on...spammers via Contact Us dropped back to its normal level. Captcha definitely helps...but still wish it was better.