mainly happens when your account is reactivated or deactivated and your bank made amendments in Financial Institution Transaction ID(FITID). Follow the steps given below to fix duplicate transactions in Quicken:
Deactivate your account: First of all, navigate to Tools and choose Account List. Next, you need to click on Edit for the account you want to deactivate. Navigate to Online Services tab and click on Deactivate. Click Yes to confirm.
Reactive your account: Firstly, click on Tools and choose Account List. After that, click on Edit and select Online Services tab. In the end, click on Set Up Now to complete the procedure.