I’ve read that mautic can update existing contacts, yet every time I attempt this, only new contacts are being created. What is the secret to success with this feature?
It can but you need to use a field that identifies the contact. Most of the time, the email ID is, by default, the unique key for the contact model. So if you use an email field in your form (means a tetx field mapoed to the email custom field), then the identification will happen and contact will be updated