How to make condition inside mutation to restrict mutation if hero dupliacates in the same draft?

I`m not able to understand how to make statement so if I make draft 1 and I have two heroes with same ID (or name, for example 'Spectre') it will restrict mutation with an error: 'You can have same hero in the draft'

Bolded text inside user_draft.py is unused because I dont know were or how to make condition. Inside mutate before I save it or?

user_draft.py

import graphene
from graphene_django import DjangoObjectType
from user_draft.models import UserDrafts
from users.schema import UserType
from heroes.schema import HeroType
from users.models import User
from heroes.models import Hero


class UserDraftType(DjangoObjectType):
    class Meta:
        model = UserDrafts


class Query(graphene.AbstractType):
    all_drafts = graphene.Field(lambda: graphene.List(UserDraftType))

    def resolve_all_drafts(self, info):
        return UserDrafts.objects.all()


class CreateDraft(graphene.Mutation):
    message = graphene.String()
    user = graphene.Field(UserType)
    hero = graphene.Field(HeroType)

    class Arguments:
        draft = graphene.Int()
        hero = graphene.ID()
        user = graphene.ID()

    @staticmethod
    def mutate(self, info, draft, user, hero):
        hero_id = Hero.objects.get(id=hero)
        user_id = User.objects.get(id=user)

        **error_message = 'You cant duplicate heroes'
        draft_error = 'You can only pick 1, 2 or 3.'**

        draft_data = UserDrafts.objects.create(
             draft=draft,
             hero=hero_id,
             user=user_id,
         )

         draft_data.save()

         return CreateDraft(
              message='You created ur hero pool successfully'
         )


class Mutation(graphene.ObjectType):
    create_draft = CreateDraft.Field()

So inside my CreateDraft mutation I want to be able to restrict user not to have same pick in draft 1 for example or draft 2, also because draft is type intiger I want to be able user to pick only 1, 2 and 3 not to have negative intiger or positive more then 3.

This is first time i need condition inside graphene mutation so I dont know how to do it. Im using posgresql. With this code I`m able to create mutation but i can duplicate heroes and also drafts.

just in case, models.py

from django.db import models
from users.models import User
from heroes.models import Hero


class UserDrafts(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="user_id")
    draft = models.IntegerField()
    hero = models.ForeignKey(Hero, on_delete=models.CASCADE, null=True)

    def __str__(self):
        return self.user.username

    class Meta:
        verbose_name = "UserDraft"
        verbose_name_plural = "UserDrafts"