Audit Log

I followed the instructions using global event to add username, ip and date to a table "Login log"when a user successfully logs in. However I find sometimes I get duplicate entries in the log table. Do you have any ideas? the code I am using is the same as your example script in a previous post with slight field name changes.
function AfterSuccessfulLogin()

{

//** Insert a record into another table ****

global $conn;

$Username = @$_SESSION["UserID"];

$IP = $_SERVER["REMOTE_ADDR"];

$strSQLInsert = "insert into `Login log` (Username, IP, datetime) values ('$Username', '$IP', NOW())";

db_exec($strSQLInsert,$conn);
}
Can you help. Thanks.