Welcome to Community, @Gosia1!
 

Let me help you delete the duplicate cheque in QuickBooks Online. Here's how:
 

1. In the left menu, select the Expenses tab. 
2. Click on Vendors. 
3. Locate the vendor that has the duplicate check. 
4. In the Transaction list, find the check you want to delete.
5. Click the drop-down beside Print Check, then choose Delete.
   
   

Please check this article on how to categorize all downloaded banking transactions to avoid duplicate records: Categorize and match online bank transactions in QuickBooks Online.
 

You can also visit this article if you want to reupload your transactions: Manually upload transactions into QuickBooks Online.
 

If you have any other concerns about managing transactions, please visit us again. We're available 24/7. Have a great day!