How do you overwrite entry in MySQL?

[eiSecure]

Okay, I have this code so far:

   $ip = getenv("REMOTE_ADDR");
   $view_date = date("F j, Y");
   echo "<br>Your IP address is $ip<br>Today's date is $view_date.";
      
   $sql = "SELECT * FROM page_view"; 
   $connection = mysql_connect("localhost", "**********", "**********") or die("Could not connect.");
   $db = mysql_select_db(**********, $connection) or die("Could not select database.");
   $result = mysql_query($sql,$connection);
   
   echo "<br>The following people have visited this website:<br>";  
   
   while ($row = mysql_fetch_array($result)) {
     $id = $row['id'];
     $username = $row['username'];
     $ip = $row['ip'];
     $view_date = $row['view_date'];
     echo "$username<br>";
    }
    
   $sql = "REPLACE INTO `page_view` (`id`, `username`, `ip`, `date`) VALUES ('', '$user', '$ip', '$view_date')"; 
   $result = mysql_query($sql,$connection) or die("ERROR2!"); 

When I run that, it displays the viewer's name correctly, but if the user returns, his name is displayed again! How would I make it so that the name of the user is shown only once?