netbox/netbox/dcim/forms/formsets.py

from django import forms

__all__ = (
    'BaseVCMemberFormSet',
)


class BaseVCMemberFormSet(forms.BaseModelFormSet):

    def clean(self):
        super().clean()

        # Check for duplicate VC position values
        vc_position_list = []
        for form in self.forms:
            vc_position = form.cleaned_data.get('vc_position')
            if vc_position:
                if vc_position in vc_position_list:
                    error_msg = f"A virtual chassis member already exists in position {vc_position}."
                    form.add_error('vc_position', error_msg)
                vc_position_list.append(vc_position)