next to the add to cart button code in the repeat region, you can add this code to show the message:
<?php if($yourCartObject->InCart($row_RecordsetName['IDColumn']) == true) echo("This item has been added"); ?>
Where "yourCartObject" is the name of your eCart object, "RecordsetName" is the name of the products recordset and "IDColumn" is the name of the ID Column in your products tabel.