"This key is already associated with an element of this collection"
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 Tranasaction Pro Importer goes to look for the Customer in QuickBooks Online, Importer does not find the Customer name in the import file in QuickBooks Online because of the extra space. Therefore, 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.