You may have duplicate customers in QuickBooks Desktop when you convert a lead. This occurs when:
Lead is created in Method.
Customer with same display name is created in QuickBooks.
Lead in Method is converted to a customer.
If you are in QuickBooks Desktop and try to convert the lead to a customer, the customer goes into a sync conflict. (In QuickBooks Online, you can't even convert the lead because it will create a duplicate customer.)
To solve this problem, you will need merge the two customers.
Rename the lead.
Convert the lead to a customer and have it Sync with QuickBooks.
Merge both customers.
If you wish to merge customers in QuickBooks Desktop, simply rename the duplicate customer, e.g. "Bob Crenshaw2" to the original name. QuickBooks will prompt you to merge them.
If you wish to merge the customers in Method, please see our article Merge Contacts and Customers.
Once you do that and it syncs, then all the transactions and all the activities, opportunities, etc., will be merged under one customer.