// This will help ensure that the values you are using are being set
echo 'DEBUG:<br />';
echo '$Id: ' . $Id . '<br />';
echo '$Current_Id: ' . $Current_Id . '<br />';
echo '<pre>' . var_dump( $_POST ) . '</pre>';
echo '<br />';

echo '<div class="video_post_bottom">';
echo '<div class="favorit">';
// You don't need to add the action if it's posting back to itself
echo '<form action="" method="post" class="favorites">';
echo '<input type="image" src="Images/Favorites.png" name="FAV" value="" />';
echo '<input type="hidden" name="FAV_Id" value="' . $Id . '">';
echo '</form></div>';

if ( isset( $_POST['FAV'] ) ) {
    // You should do some error checks here, ensure that your value is numeric, or a string, or a certain length...

    echo '<p>isset _POSTFAV</p>';
    $Video_Id = mysql_escape_string( $_POST['FAV_Id'] ); // Never use unescaped input values in an SQL query!

    // While the method of formatting your SQL statements is purely up to you, this method can ensure further data
    // integrity. Good SQL practice also involves never selecting more columns than you need, SELECT * should
    // be avoided whenever possible (and its always possible)
    $query7 = sprintf( "SELECT Video_Id FROM Favorites WHERE Video_Id = '%d' AND User_Id = '%s' LIMIT 1", $Video_Id, $Current_Id );
    echo '<p>SELECT * FROM FAVORITES</p>';

    $result_C = mysql_query( $query7 );

    // We don't need to loop through the results, or even fetch them since we know that if
    // no results were returned, the record wasn't found.
    if ( ! $result_C ) {
        echo '<p>INSERT INTO Favorites</p>';
        $query = sprintf( "INSERT INTO Favorites (Video_Id, User_Id) VALUES ('%d', '%s')", $Video_Id, $Current_Id );
        $result = mysql_query( $query );
        echo '<p>' . $Title . ' has been added to your favorites</p>'; // Not sure where $Title is coming from?
    }
    else { // No need to compare again here, if the row was found, it's in there
        echo '<p> THIS IS ALREADY IN YOUR FAVOURITES </p>';
    }

}

echo '<div class="comments"><a href="Comment/Comment.php?V_Id=' . $Id . '"><img src="Images/Comments.png"></a></div>';
echo '</div>';