cors policy blocking request from my docker django nginx app

I am trying to make a POST request from my React app deployed locally to my dockerized Djano app deployed in Google Compute Engine. I am receiving this error:

Access to XMLHttpRequest at 'http://X.X.X.X/register/' from origin 'http://localhost:3000' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header contains multiple values '*, *', but only one is allowed.

I have added django-cors to my app.

INSTALLED_APPS = [
    ...
    'corsheaders',
    ...
]
MIDDLEWARE = [
    ...
    'corsheaders.middleware.CorsMiddleware',
]
CORS_ALLOW_ALL_ORIGINS = True
CORS_ALLOW_METHODS = [
    'DELETE',
    'GET',
    'OPTIONS',
    'PATCH',
    'POST',
    'PUT'
]

CORS_ALLOW_HEADERS = [
    'accept',
    'accept-encoding',
    'authorization',
    'content-type',
    'dnt',
    'origin',
    'user-agent',
    'x-csrftoken',
    'x-requested-with',
]

This is my nginx config which I copied from https://enable-cors.org/server_nginx.html.

upstream api {
    server backend:8000;
}

server {
    listen 80;
    server_name example.edu.ph;

    location / {
        proxy_pass http://api;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        proxy_redirect off;

        if ($request_method = 'OPTIONS') {
            add_header 'Access-Control-Allow-Origin' '*';
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
            add_header 'Access-Control-Max-Age' 1728000;
            add_header 'Content-Type' 'text/plain; charset=utf-8';
            add_header 'Content-Length' 0;
            return 204;
        }
     if ($request_method = 'POST') {
            add_header 'Access-Control-Allow-Origin' '*' always;
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range' always;
            add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
        }
     if ($request_method = 'GET') {
            add_header 'Access-Control-Allow-Origin' '*' always;
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS' always;
            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range' always;
            add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
        }
    }

    location /static/ {
        alias /usr/src/app/static/;
    }
}

Is there something I am missing? How can I resolve this error and make a successful request?