Consider identity_ids as one2many field in the product template
You can write an onchange function for this field.

@api.onchange('identity_ids')
def onchange_identity_ids(self):
    for order in self.identity_ids:
        line = self.identity_ids.filtered(lambda l: l.barcode == order.barcode)
        if len(line) > 1:
            raise ValidationError(_('You cannot have multiple lines with same barcode(%s)')%(line[1].barcode))
            break

Here's an example of using an Automated Action to prevent duplicates (Odoo already blocks duplicate barcodes so this is the Internal Reference).  If I understand your requirement you could change the Python code to ensure that there are no duplicates within the product identity group.

https://odootricks.tips/automated-action-to-prevent-duplicates/

Model:  Product Template
Action To Do:    Execute Python Code
Trigger Condition:   On Creation & Update
Before Update Domain: Internal Reference is set
Apply on:  Match all records 

Python Code

if record.default_code:
   existing_product = env['product.template'].search([('id','!=',record.id),('default_code','=',record.default_code)])
   if existing_product:
     raise Warning("You can't have the same Internal Reference Number in Odoo twice!")