There's no technical reason not to have multiple accounts with the same email address. The main reason for this warning is that people commonly forget their login info, then create a new account thinking that will somehow give them access to their original account.
Confusion can also occur if, for example, you renew one account and then a short time later you get a renewal notification for the other account. Every notification like that will include the account name, so you can pay attention to that to see what account the notification is for.