counter value for LineItem duplication

Hi We have target structure with Header and LineItem. But we have to populate 4 LineItems for each Header.

So i have duplicated the other 3 Line Items using 1st Line Item and is fine.

But here I would need to send a unique value for the each Line Item.

I know this can be acived through counter, but if there are many Line Items : say 1st Line Item has come for 20times and 1st duplicated LineItem should have the value of 21 and so on.

Please suggest how we can achieve this.