Getting 'The 'Access-Control-Allow-Origin' header contains multiple values' with MEAN and nginx - node.js

Question

I have developed an application in MEAN stack and used Nginx for HTTPS.
I have used socket.io and developed rest apis in node.
Below is my Nginx configuration;
server {
# Enable HTTP/2
listen 443 ssl;
listen [::]:443 ssl http2;
server_name api.some.com www.api.some.com;
# Use the Let’s Encrypt certificates
ssl_certificate /etc/letsencrypt/live/api.some.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/api.some.com/privkey.pem;
# Include the SSL configuration from cipherli.st
include snippets/ssl-params.conf;
location / {
if ($request_method = 'OPTIONS') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,from,x-auth,Access-Control-Allow-Origin';
add_header 'Access-Control-Max-Age' 1728000;
add_header 'Content-Type' 'text/plain; charset=utf-8';
add_header 'Content-Length' 0;
add_header 'Cache-Control' 'no-cache';
return 204;
}
if ($request_method = 'POST') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,from,x-auth,Access-Control-Allow-Origin';
add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,from,x-auth,Access-Control-Allow-Origin';
add_header 'Cache-Control' 'no-cache';
}
if ($request_method = 'GET') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,from,x-auth';
add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range,from,x-auth';
add_header 'Cache-Control' 'no-cache';
}
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-NginX-Proxy true;
proxy_pass http://localhost:5000/;
proxy_ssl_session_reuse off;
proxy_set_header Host $http_host;
proxy_cache_bypass $http_upgrade;
proxy_redirect off;
##### changes for socket.io
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}
}
I am getting "The 'Access-Control-Allow-Origin' header contains multiple values" error when my application is attempting to connect socket.
I have found a workout. If I comment the 'add_header' part from the GET section and add the 'Access-Control-Allow-Origin' header from my code then it is working fine. If I don't add the header in my code then the GET rest apis are not working , its asking for the 'Access-Control-Allow-Origin' header.
But I don't want to do that as nginx should handle this, my code should not bother about these headers.
What am I missing?
UPDATE :
I added 'Access-Control-Allow-Origin' in allow headers part of the GET in nginx and removed the GET specific code from node server , the get rest APIs started working BUT the socket started throwing CORS origin error.