you can get the idea by this code .... just get unique ids of each many2one field ...in my case i had 2...

def onchange_line_course(self):

lists_course=self.courselines.mapped('field1.id')
lists_assesment=self.courselines.mapped('field2.id')
for rec in lists_assesment:
    for crs in lists_course:
        record_count = self.courseline.filtered(lambda rex: rex.course.id == crs and rex.assessment.id == rec)

       if len(record_count) >1:
 print(cannot select duplicate values


if you have only one many2one filed then only one for loop would be required

Hi,

Well you need specific development for that. It's not something you could configure.

You need to override write() of the main model and in write() you can iterate on the IDs of o2m field and put a check on the Product if it repeats and raise warning. Write() will be called when you save main form, so you won't be notified while you actually inserting line items. I am not sure onchange of o2m field would help because ID of newly inserted line item is not yet created while you add it, but you can give it onchange(o2m) also try.

Regards,