Django custom User model throwing SystemCheckError - The field 'username' clashes with the name 'username'

I'm writing a custom User model in Django, inheriting it from AbstractUser as is mentioned in the official Django tutorials. But this throws an error.

django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:

ERRORS:
redditauth.RedditUser.username: (models.E006) The field 'username' clashes with the field 'username' from model 'redditauth.reddituser'.

Here is the code I wrote for the custom user model.

class RedditUser(AbstractUser):
    username = models.CharField(unique=True, primary_key=True, validators=[validate_reddit_username], max_length=20)
    token = models.CharField(max_length=256)

    USERNAME_FIELD = username
    REQUIRED_FIELDS = ['token']

    def reddit(self):
        with open('secret.json', 'r') as f:
            secret = json.load(f)

        return praw.Reddit(client_id=secret['client_id'], client_secret=secret['client_secret'],
                           refresh_token=self.token, user_agent='Plan-Reddit by /u/SkullTech101')

I have tried renaming it to something other than username, thinking that maybe there was already field named username in AbstractUser, but that didn't solve the problem.