Hi,

Please check with this code,

def _create_delivery_order(self):

    picking_obj = self.env['stock.picking']

    warehouse = self._default_warehouse()

    destination_id = self._default_destination_id()

    # stock = self.env['stock.location'].search([

    #      ('warehouse_id', '=', warehouse.id),

    #     ('usage', '=', 'internal')

    # ], limit=1)

    # Create the picking without move lines
    picking = self.env['stock.picking'].create({
        'rma_order_id': self.id,

        'partner_id': self.partner_id.id,

        'partner_invoice_id': self.partner_invoice_id.id,

        'branch_id': self.branch_id.id,

        'picking_type_id': self._default_picking_id(),

        'location_id': self.env.ref('stock.stock_location_customers').id,

        'location_dest_id': destination_id,
        'origin': self.name,
    })
    # Track processed product IDs to avoid duplication
    copy = []
    for line in self.order_lines:
        # Check if move line for the same product already exists
        existing_move = self.order_lines.filtered(
            lambda m: m.product_id.id == line.product_id.id)
        if line.id not in copy:
            copy.append(line.id)
            # Create a stock move (not move_line) for the combined quantity
            picking.move_lines.create({
                'picking_id': picking.id,
                'product_id': line.product_id.id,
                'name': line.product_id.name,
                'price_unit': line.price_unit,
                'product_uom_qty': sum(existing_move.mapped('qunatity')),#Quantity sum
                'product_uom': line.product_id.uom_id.id,
                'location_id': self.env.ref(
                    'stock.stock_location_customers').id,
                'location_dest_id': destination_id,
                'reference_picking': line.picking_id.name,
                'rma_line_id': line.id,
            })

    if self.request_type == 'cancel_so':

        picking.name = self._sequence_delivery_order('C')

    else:

        picking.name = self._sequence_delivery_order('RET')

    return picking

Hope it helps