Welcome to the Method forum!
The stock web form validates whether a customer already exists via the Name and Email fields. If it finds a match for both the name and the email, it will create the activities and the opportunity for the existing contact. If however, the name matches, but the email does not, it will create a new lead record with (#) appended to the name since it cannot be guaranteed that this is the same customer.
The conditional statement that I included in the screenshot above is where this validation takes place. If you'd like to add your own validation for checking if the customer exists, you can add some additional conditions to the statement via an AND or an OR.
The stock web form also does not update the customer information if it finds the customer already exists. If you'd like to add this, it can be done with an Update Fields In Table action. I hope this helps.