Docker app requests blocked by CORS to another container

Question

I've got two containers, one for a frontend app (build with Sveltekit) and the another for a backend app (Laravel, Nginx, MySQL).

I'm trying to access my backend app from my frontend, but I'm getting the following error:

Access to XMLHttpRequest at 'http://127.0.0.1:8000/api/auth/login' from origin 
'http://localhost:8080' has been blocked by CORS policy: Response to preflight request 
doesn't pass access control check: The 'Access-Control-Allow-Origin' header contains 
multiple values '*, *', but only one is allowed.

xhr.js:220          POST http://127.0.0.1:8000/api/auth/login net::ERR_FAILED

Weird enough, I can access the API from my REST client, meaning that my backend app is accessible from outside the container.

Here's my Nginx config:

server {
    listen 80;
    index index.php index.html;
    error_log  /var/log/nginx/error.log;
    access_log /var/log/nginx/access.log;
    root /var/www/public;
    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass app:9000;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
    }
    location / {
        try_files $uri $uri/ /index.php?$query_string;
        gzip_static on;
    }
}

Here's my backend's docker-compose file:

version: "3.9"
services:
  app:
    build:
      context: ./
      dockerfile: Dockerfile
    image: dmc
    container_name: dmc-app
    restart: unless-stopped
    working_dir: /var/www/
    depends_on:
      - db
      - nginx
    volumes:
      - ./:/var/www/
      - ./docker/php/conf.d/xdebug.ini:/usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini
      - ./docker/php/conf.d/error_reporting.ini:/usr/local/etc/php/conf.d/error_reporting.ini
    expose:
      - "9003"
    networks:
      - dmc-net

  nginx:
    image: nginx:1.23.2-alpine
    container_name: dmc-nginx
    restart: unless-stopped
#    environment:
#      PHP_IDE_CONFIG: "serverName=dmc-server"
    ports:
      - "8000:80"
    volumes:
      - ./:/var/www
      - ./docker-compose/nginx:/etc/nginx/conf.d
    networks:
      - dmc-net

  db:
    image: mysql:8.0.31
    container_name: dmc-db
    restart: unless-stopped
    # using 3307 on the host machine to avoid collisions in case there's a local MySQL instance installed already.
    ports:
      - "3307:3306"
    # use the variables declared in .env file
    environment:
      MYSQL_HOST: ${DB_HOST}
      MYSQL_DATABASE: ${DB_DATABASE}
      MYSQL_PASSWORD: ${DB_PASSWORD}
      MYSQL_ROOT_PASSWORD: abcd1234
      MYSQL_USER: ${DB_USERNAME}
      SERVICE_TAGS: dev
      SERVICE_NAME: mysql
    volumes:
      - ./docker-compose/mysql:/docker-entrypoint-initdb.d
      - mysql-data:/var/lib/mysql
    networks:
      - dmc-net

networks:
  dmc-net:
    driver: bridge

volumes:
  mysql-data:

and here's my frontend's docker-compose file:

version: "3.9"

services:
  dmc-web:
    build:
      context: .
      dockerfile: Dockerfile
    container_name: dmc-web
    restart: always
    ports:
      - "3000:3000"
      - "3010:3010"
      - "8080:8080"
      - "5050:5050"
      - "24678:24678"
    volumes:
      - ./:/var/www/html

what am I missing? why can I access my backend from my REST client but not from my frontend app?

Thanks