Can I prevent duplicate Customers/Vendors from being created?

Yes – the QuickBooks Online API exposes the QuickBooks ID field for Customers and Vendors which is their unique identifier for a Customer or Vendor (Supplier) record. We store this in a field in the fmAccounting Link file in the Customers and Vendors tables (same for Invoices, Bills, Purchase Orders etc – they all have their own unique QuickBooks ID). You can use the fmAccounting Link file to download all existing Customers and Vendors from QuickBooks Online to get access to the QuickBooks ID values and then create a field in your FileMaker solution to store this, then populate this from the fmAccounting Link file via a relationship based on Company Name etc.

Once this is populated any future uploads to QuickBooks Online will not create a duplicate Customer or Vendor in QuickBooks Online but use this to update an existing Customer or Vendor.