Try with this code.

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def merge_duplicate_product_lines(self, res):
for line in res.order_line:
if line.id in res.order_line.ids:
line_ids = res.order_line.filtered(lambda m: m.product_id.id == line.product_id.id)
quantity = 0
for qty in line_ids:
quantity += qty.product_uom_qty
line_ids[0].write({'product_uom_qty': quantity,
'order_id': line_ids[0].order_id.id})
line_ids[1:].unlink()

@api.model
def create(self, vals):
res = super(SaleOrder, self).create(vals)
res.merge_duplicate_product_lines(res)

return res

def write(self, vals):
res = super(SaleOrder, self).write(vals)
self.merge_duplicate_product_lines(self)
return res