to check if an item is in the cart, you need to know the ID for the item, for example, if the ID is 12, you could use the following if statement:
if($<yourCartName>->InCart("12") == true)
you could show how many of item 12 are in the cart using this code:
$<yourCartName>->ConditionalTotal("Quantity", "ID", "12")
to show your custom message, the fill code would look like:
<?php if($<yourCartName>->InCart("12") == true) { ?>
You already have <?php echo($<yourCartName>->ConditionalTotal("Quantity", "ID", "12")); ?> of this item in your cart.
<?php } ?>
NOTE: in this sample code, you need to replace "<yourCartName>" with the name of your eCart object.