Does Soapbox Mailer prevent sending to the same Campaign Member in a Campaign twice?

Yes and no. After Soapbox Mailer sends an email to a Campaign Member in your Soapbox Mailer campaign, it marks the Campaign Member's status as one of the following:

• Sent
• Rejected

If you add more Campaign Members to this same campaign with a status of "Targeted", or update the status of any existing Campaign Member to "Targeted", Soapbox Mailer will send only those Campaign Members the email the next time you click the Send button.  Campaign Members with any status other than "Targeted" will not have the email sent/resent to them.