MESSAGE

"This key is already associated with an element of this collection."

RESOLUTION

Check the import file to see if there are extra spaces in the list name (Customer, Vendor, Employee, or Product and Service) that is being added.  

The issue is most likely caused by the scenario below:

Customer name in the import file is RIGHT  WAY CLEANERS (note the extra space between the first and second word).

Customer already exists in QuickBooks Online as RIGHT WAY CLEANERS

When Rightworks Transaction Pro Importer searches for the Customer in QuickBooks Online, it cannot find the Customer name in the import file because of an extra space. Therefore, the importer then tries to add the Customer. The problem occurs because QuickBooks Online strips the extra spaces when adding a new Customer. 

Therefore, remove any extra spaces from the import file and retry the import.