Avoid duplicate product name in Cart

I'm selling downloadable products on my website. Sometimes, by mistake a customer adds the same product twice and pays for it. As a result of this mistake each time i need to refund the payment to customer for duplicate product.

Is there any way i can put a check on shopping cart to allow unique products only. The client must not be able to add same product twice.

Please tell me how can i do that?

Admin > Catalog > Categories/Products > edit your product > Product Qty Maximum: [] 0 = Unlimited, 1 = No Qty Boxes
Set to 1.

You could use this add-on: http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=1477.

I wrote it because I had a similar problem with my downloadable products.

Huh... so the max does not apply to downloadables? Obviously I have never used them.

Glenn, your answer should be sufficient.

I have tried it:

Admin > Catalog > Categories/Products > edit your product > Product Qty Maximum: [] 0 = Unlimited, 1 = No Qty Boxes
Set to 1.

But the problem is still there i can add same product twice in the shopping cart. Is there any other solution?

I'm having trouble duplicating that on a clean fresh uncustomized install of v1.3.9h. Setting "Product Qty Maximum" to 1 prevents me adding the item to the cart multiple times. I get an error message instead.

You may be right because i have customized the "product info page" and i might have removed the quantity check code from the "tpl_document_product_info_display.php" file. I think i'll have to look for the original file and add the code for "quantity check" again. Is that right?

But you know the "Product Qty Maximum" was set to 0 previously and now setting back it to 1 does not work for me.

What do you say?

If you have removed some of the built-in safeguard checks, of course you may have a problem that we can't duplicate.

fawad123:

But you know the "Product Qty Maximum" was set to 0 previously and now setting back it to 1 does not work for me.

What do you say?
I say the problem is on your end, since the test I did started with it set to 0, and when I changed it to 1 it prevented me from adding more than 1 of that item to my cart, exactly as intended.

Thanks for taking me to the right direction. I"ll update the code in the doucment product info page.