Client: “I’m trying to register on behalf of someone else but it’s saying my email address is already in use!”

Me [internally]: “Wait, you seriously thought you could register multiple accounts with the same email?”

Ouch.

Welcome to devRant!

Really good 😁

Well, you can, if you do the "+" trick.
Example: emails sent to foo+1@bar.biz will be sent to foo@bar.biz (tested with gmail).
Applications don't usually test for these, so you can have multiple accounts with the "same" email.