[Answer]-Django form validation on create view only

def clean_tag_number(self):
    tag_number = self.cleaned_data['tag_number']
    qs = self.Meta.model.objects.filter(tag_number=tag_number)
    if self.instance:
        qs = qs.exclude(pk=self.instance.id)
    if qs.count() > 0:
        raise forms.ValidationError('You already have a calculation based on that tag number, please choose another.')

    return tag_number

Thanks for the posts and elaborating on Daniels answer below I have the following working code.

def clean_tag_number(self):
    tag_number = self.cleaned_data['tag_number']
    if self.instance and self.instance.pk == None:
        try:
            Calc.objects.get(tag_number = tag_number)
        except Calc.DoesNotExist:
            return tag_number
        except Calc.MultipleObjectsReturned:
            raise forms.ValidationError("You already have a calculation based on that tag number, please choose another.")
        raise forms.ValidationError("You already have a calculation based on that tag number, please choose another.")
    else:
        return tag_number

This is the first time Ive written any validation so please feel free to point out any issues.