An already registered email address does not necessarily mean that it's registered with your locations, it might be registered with a different operator even if their account is no longer active there.
You can use the same email address for every account by following this workaround, you will just need to add a "+" and any extra word to the portion before the email domain on the account. That way, the system can see that the email is not being reused, and at the same time, the customer will receive all the notifications on the same original email without needing to create any new ones.
So, for example, if the email they want to use is support@spheremail.co, you can add the email on the account using these versions of the same email: support+spheremail@spheremail.co - support+team@spheremail.co - support+anything@spheremail.co... etc.