Hello there, Lexie22.

Yes, you can add the sales payments from the Banking page even you have not entered yet in QBO. This way, it will not duplicate the transaction and it will automatically reflect on the dashboard. Just make sure that you have categorized them properly.

Please refer to this article for a complete instruction manually upload transactions into QuickBooks Online.

All downloaded transaction will show on your For review tab. To avoid duplicates, you can exclude the double transactions from there.

Once done, you can start matching your downloaded bank transactions. 

If you meant something else, please let me know and I'd be happy to help you further.