Hello there, JoanChanning.
QuickBooks uses both amount and transaction dates when looking for a match instead of transaction numbers. You'll see multiple possible matches if there are several transactions with the same amounts that are within 90 days.
When finding a match, you have the option to view the details of the transaction you entered into the system. Make sure this is the correct match by reviewing the Deposit to, Payment method, and Bank account fields on forms for the existing transaction in QuickBooks.
It can be that the bank downloaded the transactions twice. You'll want to verify through logging into your bank's website for any possible alerts or notifications. However, if the previously matched transactions are correctly done. You can exclude those that were downloaded again so you don't have to undo and redo the reconciliation.
To give you more details on how QuickBooks handles banking transactions, please see this link: Categorize and match online bank transactions in QuickBooks Online.
Let me know if you have other questions. Take care and have a good one!
