Multiplied visits

Hello; I remember it's not the first time it happens to me, and I want to ask if others have found this phenomenon: Under concrete circumstances (I don't know which are these, but everytime is the same movement), a PHP page only visited one time it's executed from 2 to 4 times. There is no frames, and no other HTML references to same script. In my case it's a simple form to which the "submit" button is pressed; the PHP file is executed 2 times with Midori web browser (Webkit) or 4 times with Firefox browser (Gecko). Only first execution receives form data (POST); next executions don't receive them, and it happens that at the forth execution (Firefox) HTTP_REFERER is not reveived, asif it was the first visit. This is the form: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html lang="ca"> <head> <title>TePeVe</title> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <meta name="robots" content="none"> <meta name="description" content="Message about problems"> <meta http-equiv="Content-Style-Type" content="text/css"> <base target="_self"> </head> <body> <form name="for_newsession" action="." method="post"> <input type="hidden" name="ges" value="newsession"> <input type="hidden" name="sid" value="1ec088cc439b68230cf93d930f627593"> <input type="hidden" name="pcl" value=""> <input type="hidden" name="created_user" value=""> <p>Nom d'usuari: <input type="text" name="login" value="" size="16" maxlength="32"></p> <p>Contrasenya: <input type="password" name="password" value="" size="16" maxlength="32"></p> <p><input type="text" name="created_balance" value="" size="8" maxlength="16"></p> <p><input type="submit" name="submit" value="Comença"></p> </form> </body></html>