There is a site I use that does this. I try and use [email protected] as the email for most things I sign up to. Occasionally this gets wonky like the site in question because it was originally a Patreon sub for access so the email for the site was [email protected] then they switched to their own payment system but my email for them is now locked to [email protected].

So when I log in with [email protected] it thinks it's new account and just signs you up as a new user rather than saying you don't have an account.

Now imagine having a bad day and you try and sign in with 5 different emails because you forgot wha one it was and snow you have 5 different "new account" flows with all the bullshit +12+24+48 hour onboarding/retention/marketing emails for all 5 email addresses.

My fault entirely obviously but it is a little mad.