dates_list = []
for form in self.forms:
    start = form.cleaned_data.get('start')
    for day in xrange((form.cleaned_data.get('end')-start).days):
        current_day = start + datetime.timedelta(days=day)
        if current_day not in date_list:
            date_list.append(current_day)
        else:
            form._errors['end'] = form.error_class([
                "Overlaping intervals"
            ])
            break


:)