Hi Tomas,
Your question may have been answered through another resource. For those interested in the answer see below.
When setting up your web form or form capture you have the ability to create a new visitor as a Lead or a Contact. Since we use the Email Address as the unique identifier, it is not possible for a someone who exists in the system to submit a form and have the system create a new lead every time.
As for the second question you can evaluate the Posted Form records under the Related Dropdown Menu for the lead or contact and compare that to the created on date for the lead/contact. Please let us know if there are any additional questions.