Currently in order to identify a form in password_generation_agent.cc, the following structure is used

struct PasswordFormGenerationData {
  // The name of the form.
  base::string16 name;

  // The action target of the form; this URL consists of the scheme, host, port
  // and path; the rest is stripped.
  GURL action;

  // Field in which password should be generated.
  FormFieldData generation_field;
};

And if a page has more than one form with the same name and action the first form will be taken for generation. The example is a page https://torrentz.eu/profile which contains both sign-in and sign-up forms, with the same name and action and since sign-in forms is the first in DOM, generation pop-up is shown on it. 

One of the possible simple solution is to check number of fields for forms identification.