Hello, @Laura_C.

I’m here to share some insights about your bank feeds downloading existing transactions and causing duplicates in QuickBooks Online.

Since QBO is dependent on what your financial institution shares with us, it’s possible that the bank is experiencing some technical difficulties. This would be the reason you're getting duplicate transactions. You may want to reach out to your bank and verify this information. 

To resolve this, you can remove the duplicates by excluding them from the Banking page. Here’s how:
1.    Select Banking from the left menu.
2.    In the For Review tab, put a check mark on the transactions you want to exclude.
3.    When done, go to the Batch Actions drop-down and click Exclude Selected.

For more information, check out this article: https://quickbooks.intuit.com/community/Banking-and-bank-feeds/How-to-exclude-expenses-from-download....

That should do it. Feel free to leave a comment below if you need additional assistance. I’m always here to help.