Hi @MWanson hope you are doing well.
When a contact that you already have in the CRM submits a new form with info
that info gets updated (you can always see the history in the property)
So if its using the same email, no duplicate 'll be created.
You can opt to show the contact the information you already have
with this option in your form
for them to confirm or change it.
See this example: check that Yellow is off and red is on
Form info submission_HubSpot_Andimol
Hope this helps
Have a good day!