Message Mapping - assign value of duplicate items

Dear All,

Need your advice to sort out below.

below is the structure I have defined in Message Mapping.

Logic

If Order Item - J_3APOSNR is "Blank" then item_key = "some value".

I defined my mapping as below

this is working fine as long as I do not pass values to "J_3APOSNR"

But If I set some values to "J_3APOSNR" and repeat "Order Item" header node more than one , showing an error that item_key cannot be created.

thanking all in advance