Sure thing, this is what I gathered from your question.
from django.core.exceptions import ValidationError
class MyPlugin(CMSPlugin):
title = models.CharField(max_length=200)
is_draft = models.BooleanField(default=False)
def clean(self):
manager = self.__class__.objects
self.is_draft = self.placeholder.page.publisher_is_draft
if manager.filter(title=self.title, language=self.language, is_draft=self.is_draft).exists():
raise ValidationError("Duplicate !!!")