I can guide you with fixing the issue you're having to sync your bank transactions to your QuickBooks Desktop for Mac, @Chelsea123.

First, make sure that you have the latest release for your QuickBooks Desktop for Mac. You can install the update from the QuickBooks menu. Check this article for detailed guidance: Update QuickBooks Desktop to the latest release. Then, create a backup of your QuickBooks company file to use the file in case you run into some problems. 

Once done, make sure the financial institutions' list is up-to-date. You can do it by deleting the fidir.txt file from the Library or Applications. See this article for detailed guidance: Fix online banking issues in QuickBooks Desktop for Mac. You can also follow the next solutions provided in this article if the first one didn't work. 

After downloading transactions from your bank and you already have similar entries in your register, here's how you can add and match those transactions to avoid duplicates: Add and match transactions in QuickBooks Desktop for Mac.

Let me know if you need further help with syncing your bank and QuickBooks. I'm here to assist. Have a wonderful day!