To avoid duplicates being created in your QuickBooks account, it's best to sync your QuickBooks Customers and Vendors with Syngency.

This assign which Customers and Vendors in QuickBooks correspond to a specific Contact or Talent in Syngency. If they don't yet exist in Syngency, their information can be imported from QuickBooks to create a new Contact or Talent in Syngency.

Once you have connected your QuickBooks account to Syngency, click the Sync Customers & Vendors button in the bottom left of the Settings > QuickBooks tab:

This will open the Sync Customers & Vendors window, which will show a list of your Customers and Vendors from QuickBooks on the left, and a dropdown list of Contacts or Talent in Syngency, which you can assign them to:

If you already have a Syngency Contact/Talent for the QuickBooks Customer/Vendor, you should associate them by selecting the Contact/Talent from the list, as shown below: 

If the Customer/Vendor does not yet exist in Syngency, and you would like to import their details from QuickBooks to create a new Contact/Talent in Syngency, select Create New from the list:

If you do not want to sync the QuickBooks Customer/Vendor with Syngency, simply leave the dropdown list set to Skip.
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Once you have made your selections, click the Sync button, and after a few moments, Syngency will display the result of the sync process:

Note: You should only have to run this process once, when you first start using QuickBooks with Syngency. As you create new Contacts and Talent in Syngency, they are automatically synced with QuickBooks.