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.

People do go back to the signup page a lot. Redirecting them when detecting an existing account from the email is enough in the most common cases.

Users doing "clever" stuff like a random email each time, will also probably understand the situation when they go write down which email for which domain they used and see another entry next to it.

There will be some numbers of users that can't be caught and will create many new accounts, but if thwy use your seevice that much, it will probably stop at one point.

Nothing is foolproof, but there can be many ways to mitigate the issue.