“The name of the list element is already in use. Status code = 3100. Severity = Error” message while posting to QuickBooks

This is a QuickBooks error, not CPM.  You will need to look up the consumer that is creating the error to see if he/she is duplicated in QuickBooks.  

Possible causes:

• The social security number (SSN) may be missing in QuickBooks for a consumer.
• A different SSN in CPM than what is found in QB for the employee.  Double check both SSN’s to see if they match.
• The consumer is marked in QB as inactive. This can happen when the consumer goes from active to inactive status and back to active.  Check All Employees, by selecting from the dropdown in QB, to see if the consumer has an ‘X’ by their name.