Why are my member and contact names formatted incorrectly?

If names are formatted incorrectly in Salesforce or Mailchimp merge fields so that the first name field contains both first and last names (i.e. Diana Prince Prince), you likely mapped the full "Name" field in Salesforce to the First Name merge field in Mailchimp. This can be mapped via Field Mappings (Salesforce to Mailchimp) or one of your Data Wizards. If enabled, mappings in the other direction may further compound the issue. 

You will need to:

1. Update your mappings
   • Field Mappings
     • Salesforce to Mailchimp direction - remap the First and Last Name fields on contact and/or lead to the First and Last Names in Mailchimp
     • Mailchimp to Salesforce - check you aren't mapping the Salesforce First and Last name fields to a custom "Full Name" or similar mergefield in Mailchimp, incorrectly.
   • Data Wizard - Add the First and Last Name fields to your source report, list view or query. Remap the First and Last name columns on the source to the First and Last Name Mailchimp fields.
2. Clean up the malformed data in Salesforce, so that it won't continue to reintroduce the bad data to Mailchimp. 
3. Resync with the correct mapping and data
   • Mappings can be retrospectively applied
   • Data wizards can be re-run.

More on troubleshooting mappings