Instance Already Exist Error?

HI Expert,

I have created custom business object eg:

businessobject A {

[AlternativeKey]       [Label("Order ID")]      element OID:ID;

node Batch[0,n]{

[AlternativeKey]       [Label("Item ID")]       element IID             :ProductInternalID;

                              [Label("Date")]           element ExpDate     :Date;

                        }

}

Now i am creating entry in BO.

First Entry:

When I am trying to enter second record. It throws error "instance already exits".

Second Entry:

Can any body help me. Why this error is coming and how can i solve this issue.

Thanks,

Nikhil.