How to avoid users to go back and have duplicated records?

I've searched the forms already and I see that the only way to implement conversion tracking code is by redirecting users to a Thank You page after form submission. The problem with that configuration is that our site visitors hit "back" on their browsers after they hit the Thank You page and that causes duplicate submissions on all of our forms which leads to duplicate entries in our CRM, too.

Is there any way to have the Thank You page open in a new browser window so that the user cannot hit the back button? Thank you!

Unfortunately, it's not possible to open the thank you page in a new tab or window. You may alternatively, disable the browser back function on your custom thank you page adding some JavaScript code. 

Here is a link that may help you doing so: https://www.aspsnippets.com/Articles/Disable-Browser-Back-Button-Functionality-using-JavaScript.aspx 

I hope this helps.