from odoo import models, fields, api

from odoo.exceptions import ValidationError

class YourModel(models.Model):

    _name = "your.model"

    field_one = fields.Many2one('res.partner', string='Field One')

    field_two = fields.Many2one('res.partner', string='Field Two')

    @api.constrains('field_one', 'field_two')

    def _check_same_person(self):

        for record in self:

            if record.field_one and record.field_two and record.field_one == record.field_two:

                raise ValidationError("You cannot select the same person in both fields.")

Hi,

Suppose you want to check two partner fields in sale order model , you can do as follows:

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

class SaleOrder(models.Model):
    _inherit = 'sale.order'

    partner_id_1 = fields.Many2one('res.partner', string="Partner 1")
    partner_id_2 = fields.Many2one('res.partner', string="Partner 2")

    @api.constrains('partner_id_1', 'partner_id_2')
    def _check_same_partner(self):
        """
        Constraint to ensure that the same partner is not selected in both Partner 1 and Partner 2 fields
        """
        for order in self:
            if order.partner_id_1 and order.partner_id_2 and order.partner_id_1 == order.partner_id_2:
                raise ValidationError("You cannot select the same partner in both fields.")

Replace your model and fields that you are using 

Hope this will help you

Thanks