If I link a form with Constant Contact to capture email addresses, will it detect duplicates or will it potentially create duplicates?

Constant Contact will try to create a new contact each time the form is submitted unless  you choose the settings to allow users to choose whether or not they are added. 

What you can do, is only allow for one submission per used based on their IP/Cookies or a unique questions:

https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

Set the unique question to an email field and it will only allow for one submission per email address.