HI Jacqueline,
Are you sending this through the engagement engine or do you just want to send a batch of emails to a group of people.
If you send a batch of emails through a smart campaign, there is nothing preventing the leads from receiving the message again. If you are doing it through the same smart campaign you sent through originally, you will need to go into the schedule tab, and change the smart campaign settings to your preferences (ex. everytime, every 2 weeks or what ever your preference is)
If you are sending through the engagement engine, you can clone the template and it is recognized as a new email so it will send agian.
Let me know if that helps!
-Ryan