Define  @api.constrains('customer_id','product_code')  definition in this definition you can write a logic

Maybe this add-on can help?  

https://apps.odoo.com/apps/modules/11.0/customer_product_code/

Hello

use below code.

@api.one
@api.constrains('customer_id', 'default_code') # customer_id is field and default_code is product code.
def _check_customer_product_code(self):
    if self.customer_id and self.default_code:
        code_lst = self.search([('customer_id', '=', self.customer_id.id)]).mapped('default_code')
        if len(code_lst) != len(set(code_lst)):
            raise Warning("Your error msg.")

thank you for your reply 

but unfortunately, I am new and completely non technical, and don't know python coding. 

i can just change or amend minor codes in field/model by activating developer mode. 

can you please explain it in detail?

thank you