Limit on sending double opt-in confirmation mails

When registering twice (or more) with the same email address for the same list, multiple confirmation mails are being send when double-opt in for the list is turned on. This behavior can be exploited to send hundreds of emails to the same email address. This seemed to have happened at my Listmonk instance maybe as an attempt to flood someone else's mail box. The Altcha gives a little protection for keeping out bad actors, but maybe setting limits on the amount of confirmation mails within a timespan to the same email address also helps.