I have been using quicken since before you were able to download bank transactions. I always entered everything manually. If I set up to download will it duplicate transactions. My stuff goes back over 20 years.
Thank you for reaching out to the Quicken Community with this question.
There is a possibility for duplicates due to never having used Online Banking Services before, however, most banks only allow for the last 90 days worth of transactions to download. You also have the ability to set up your registers to Review and Accept transactions before they are added. You can do so by navigating to Edit > Preferences > Downloaded Transactions > uncheck "Automatically add to banking registers > OK. You would want to make sure this is done before initiating downloads.
I hope this helps!-Quicken Jasmine0