Why invoice number is not unique?

[sushant]

Hi, 

As per the information given in the link, invoice number is a unique identification value assigned to a transaction by a merchant.

If customer pays amount through hosted page and that transaction gets declined then customer can pay through another credit card for same invoice.

So how can I maintain it as unique?

[rajvpate]

The field is meant for the merchant to have a link to ANet and is not validated by ANet.

Try generating a unique identifier for each transaction and passing it in the request.