There are several different warning messages you may have received when you attempted to sync your invoices to Quickbooks:
The invoice by this number already exists in QuickBooks Online
This is a message from QuickBooks stating there is a document in QuickBooks (either an invoice or credit memo) already with the invoice number. If it's an invoice number for a different client (created in QuickBooks), the invoice in 17hats will need to be redone with a new invoice number. If it's a credit memo, you should be able to edit the credit memo in QuickBooks and change the number.
Forcing an invoice to sync from 17hats will overwrite whatever QuickBooks Online has in their system.
SyncError in Intuit Data Services
This is an issue on QuickBooks Online's side, and is usually temporary. Please try again later, and if the issue persists, please contact us at firstname.lastname@example.org.
The name supplied already exists: Another product or service is already using this name. Please use a different name. Check your inactive Item List
This message suggests you are trying to create a product that already exists in QuickBooks. This may mean that you have duplicate products and services in Quickbooks. Please try using a different name for this item.
This error means that there was an issue in the connection between 17hats and Intuit. Intuit Data Services failed to respond to 17hats within a set time. This can result from Intuit experiencing an increased amount of traffic or issues with their servers. Unfortunately, there is nothing the 17hats team can do about this issue, so please be patient and try again at a later time.
* This feature may not be available on all plan types.