Email Marketing is set up to avoid sending duplicates of the same message.
If it is a general message (no Replacement Tags) we automatically remove any duplicate emails across your lists you are sending to. As long as you don’t send out the message, and then send out the same message again later, it shouldn’t send the message more than once to people.
If the message is using Replacement Tags, and is therefore different for each registrant, then each person will get one who registered at that email address. This would not be a duplicate message, because the Replacement Tags would make the message unique to each recipient.
Also, if you’re sending to multiple years worth of registrants and using the Replacement Tags, then each participant would still only get one copy of the email because the tags would make the email the same for them (assuming they used the same name) for each year, and those would not be duplicated either.