Docker: Admin user is created twice

#01

django.db.utils.IntegrityError: duplicate key value violates unique constraint "auth_user_username_key"
DETAIL:  Key (username)=(admin) already exists.

#02

@ -13,7 +13,12 @@ if [[ -z ${SUPERUSER_NAME} || -z ${SUPERUSER_EMAIL} || -z ${SUPERUSER_PASSWORD}
         SUPERUSER_PASSWORD='admin'
         echo "Using defaults: Username: ${SUPERUSER_NAME}, E-Mail: ${SUPERUSER_EMAIL}, Password: ${SUPERUSER_PASSWORD}"
 fi
-echo "from django.contrib.auth.models import User; User.objects.create_superuser('${SUPERUSER_NAME}', '${SUPERUSER_EMAIL}', '${SUPERUSER_PASSWORD}')" | python manage.py shell
+
+python manage.py shell --plain << END
+from django.contrib.auth.models import User
+if not User.objects.filter(username='${SUPERUSER_NAME}'):
+    User.objects.create_superuser('${SUPERUSER_NAME}', '${SUPERUSER_EMAIL}', '${SUPERUSER_PASSWORD}')
+END