Prova cosi:

enter.php

<?php
session_start();
//includo il file di Configurazione
include "config.php";
//se esiste la variabile $_SESSION['username'] è loggato

if(isset($_SESSION['username']){
$sql="UPDATE
messaggi SET user='".$_SESSION['id_user']."',
gender SET user='".$_SESSION['gender']."',
year SET user='".$_SESSION['year']."',
month SET user='".$_SESSION['month']."',
day SET user='".$_SESSION['day']."',
country SET user='".$_SESSION['country']."',
test_arousal_photo1 SET user='".$_SESSION['test_arousal_photo1']."',
test_valence_photo1 SET user='".$_SESSION['test_valence_photo1']."',
test_arousal_photo2 SET user='".$_SESSION['test_arousal_photo2']."',
test_valence_photo2 SET user='".$_SESSION['test_valence_photo2']."',
test_arousal_photo3 SET user='".$_SESSION['test_arousal_photo3']."' ,
test_valence_photo3 SET user='".$_SESSION['test_valence_photo3']."',
test_arousal_photo4 SET user='".$_SESSION['test_arousal_photo4']."',
test_valence_photo4 SET user='".$_SESSION['test_valence_photo4']."',
test_arousal_photo5 SET user='".$_SESSION['test_arousal_photo5']."',
test_valence_photo5 SET user='".$_SESSION['test_valence_photo5']."',
test_arousal_photo6 SET user='".$_SESSION['test_arousal_photo6']."',
test_valence_photo6 SET user='".$_SESSION['test_valence_photo6']."',
test_arousal_photo7 SET user='".$_SESSION['test_arousal_photo7']."' ,
test_valence_photo7 SET user='".$_SESSION['test_valence_photo7']."',
test_arousal_photo8 SET user='".$_SESSION['test_arousal_photo8']."',
test_valence_photo8 SET user='".$_SESSION['test_valence_photo8']."',
test_arousal_photo9 SET user='".$_SESSION['test_arousal_photo9']."',
test_valence_photo9 SET user='".$_SESSION['test_valence_photo9']."',
test_arousal_photo10 SET user='".$_SESSION['test_arousal_photo10']."',
test_valence_photo10 SET user='".$_SESSION['test_valence_photo10']."',
WHERE username='".$_SESSION['username']."'";
$query=@mysql_query($sql) or die (mysql_error());
echo "Benvenuto ".$_SESSION['username']." hai effettuato il login con successo
Puoi effettuare il <a href=\"logout.php\">logout</a>.";
}
//Altrimenti non è loggato
else
{
echo "Non hai fatto il login correttamente, mi spiace <a href=\"index0.php\">riprova</a>";
}
?>


Perchè usi due pagine di form login uguali?