If I added transactions manually in the check register, when linking to the bank, if I choose Add will it show as a duplicate transaction in register?

Hi there, springtriclubtre. 

Let me share some insight about the duplicate transaction.

To answer your question, they will not be duplicated. Transactions from the QuickBooks Bank Feed will not automatically add to the firm's general ledger in QuickBooks Online. 

QuickBooks will scan your data looking for matches, rules, and recognized transactions. As you review them, you can match the transactions against existing transactions in QuickBooks.

Transactions added or matched are automatically marked “cleared” in the register after you accept them. 

You can use the Find Match tool and turn on Suggested Matches to see options that might match. Or, you can use filters/searches to locate transactions. Here's how:

1. From the Banking menu, select Banking and go to the For review tab.
2. Click the transaction n and select Find a match. If you can't find the match, click Find other matches.
3. Turn on the Suggested Matches button. Then, look for the match transactions and check their boxes.
4. Select Save.

For more information about the process of matching bank transactions, check out this article: Categorize and match online bank transactions in QuickBooks Online.

To avoid duplicates, you can exclude the downloaded transactions from your bank. For more insights about the process, please check out this article: Exclude a bank transaction you downloaded into QuickBooks Online.

For additional resources about managing your banking transactions, you can check out the topics from this link: Find help with bank feeds and reconciling accounts for QBO.

Drop me a comment below if you have any other questions about matching transactions in QuickBooks. I'll be more than happy to help. Have a great day.