Below are some options to help you accomplish this:
First, you could leverage an integration tool such as Zapier or Integromat as middleware sitting between Typeform & Airtable. You could configure a scenario in either case that would do the logic of first searching for a record with that email address to see if it exists & update if it does; otherwise insert the record normally.
Another option is to let the data flow to Airtable as is in all cases, but have the target table act not as the final set of data, but rather as a processing queue. Then, use the Automations feature in Airtable to trigger an automation that essentially does the same logic as in the option above–checking to see if the email address exists & updating if so, otherwise inserting.
Depending on your use case the logic may differ a bit, but this general scenario is common in data processing & is many times referred to as an “upsert” (update if exists/insert if doesn’t).
Hope this makes sense!