If you want a certain message to be sent to users only once without any further repetitions, please add the following conditions to the user segment:
*The Event Type may vary in case you are referring not to email campaign but any other campaign type.
Your current campaign ID
from beginning of time to dispatch
This condition ensures that the message will be sent only to those users who have received 0 emails (namely, never received an email) from the current campaign since the campaign has been launched.