The scenario is we have 2 or more lines with the same product, same cost, same quantity so the total price also same.

now what we have, i create another invoice with the  previous quantity and previous amount. which means the previous invoice values is it's previous amount and quantity. Now my problem is if i create the invoice with the same product and cost and etc... as same. i can't get each line seperately because i can't differentiate the line with this scenario.

Note: With the different product my expectation working fine. but, same product it's not. 

My Code is:

@api.one
@api.depends('invoice_id.former_invoice_id')
def _get_previous_qty(self):
    if self.invoice_id.former_invoice_id:
        lines = self.invoice_id.former_invoice_id.invoice_line_ids.filtered(lambda r: r.product_id.id == self.product_id.id and r.price_unit == self.price_unit)
        self.prev_qty = sum(lines.mapped('total_qty')) 

Any suggestions for me?