Marketo: New contact created when submitting form in same browser session

Description

Steps to reproduce

1. Submit a Marketo form with valid values for FirstName and Email.
2. Re-submit the form on the same browser session (no clearing cache) with a different email address.

Expected:
After step 2, no new leads are created. Only the lead created in 1st step is updated.

Actual:
2 leads are created.