Hi there, @John_Duncan.
I'd be glad to give you some insights about working in QuickBooks Online bank feeds.
Duplicates happen when you add imported transactions that you already created in the program. To avoid duplicate deposits, you'll need to find a match for this transaction. Let me guide you on how.
To exclude the duplicates that you've added, follow the steps below.
For more details about managing your bank feeds, check out this link: Assign, categorize, edit, and add your downloaded banking transactions.
Let me also add an article for more information about reconciling an account: How to Reconcile an Account in QuickBooks Online?
If there's anything else that I can help you with, please let me know in the comment section down below.