When setting up Soapbox Mailer, one of the steps you'll work through includes the creation of a default Campaign Member record type. The purpose of doing this is to ensure you have at least one record type on your Campaign Member object that is not the Soapbox Mailer record type that will be created by Soapbox Mailer.
When working through the step titled "Create a new default Campaign Member record type if you do not already have one", you might receive the following error message from Salesforce:
"duplicate values not allowed"
This appears because you likely already have a record type for the Campaign Member record called "Default". If this is true, you can skip this step (regarding the creation of a default Campaign Member record type), since you already have one.