Hi,

Please refer to the code below:

from odoo import api, models
from odoo.exceptions import ValidationError

class StockMoveLine(models.Model):
    _inherit = 'stock.move.line'

    @api.onchange('lot_id')
    def _onchange_lot_id_serial_validation(self):
        for line in self:
            if not line.lot_id or not line.picking_id:
                continue

            # 1. Check for duplicate in same picking
            duplicate_lines = self.env['stock.move.line'].search([
                ('picking_id', '=', line.picking_id.id),
                ('lot_id', '=', line.lot_id.id),
                ('id', '!=', line.id)
            ])
            if duplicate_lines:
                raise ValidationError("This serial number is already used in this delivery note.")

            # 2. Check if serial is used in any other *validated* delivery
            other_pickings = self.env['stock.move.line'].search([
                ('lot_id', '=', line.lot_id.id),
                ('picking_id', '!=', line.picking_id.id),
                ('picking_id.state', '=', 'done')
            ], limit=1)
            if other_pickings:
                raise ValidationError(f"The serial number '{line.lot_id.name}' is already used in another completed delivery.")

    @api.constrains('lot_id', 'picking_id')
    def _check_serial_number_constraints(self):
        for line in self:
            if not line.lot_id or not line.picking_id:
                continue

            # Same logic as onchange, repeated for backend safety

            duplicate_lines = self.env['stock.move.line'].search([
                ('picking_id', '=', line.picking_id.id),
                ('lot_id', '=', line.lot_id.id),
                ('id', '!=', line.id)
            ])
            if duplicate_lines:
                raise ValidationError("This serial number is already used in this delivery note.")

            other_pickings = self.env['stock.move.line'].search([
                ('lot_id', '=', line.lot_id.id),
                ('picking_id', '!=', line.picking_id.id),
                ('picking_id.state', '=', 'done')
            ], limit=1)
            if other_pickings:
                raise ValidationError(f"The serial number '{line.lot_id.name}' is already used in another completed delivery.")

Hope it helps.