OK, I added a separate server block that looks like this:

server {
listen 80;
servername www.streamfeed.com;
return 301 http://streamfeed.com$requesturi;
}

And restarted nginx (sudo service nginx restart), but it’s still happening - I can have two tabs open to different sessions at streamfeed.com and www.streamfeed.com. Do I need to delete the www CNAME record from my DNS here on Digital Ocean now?

I sshed into our ip as root and added it to etc/nginx/sites-available/default. Prior to my changing it, the server block looked like this:

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    root /usr/share/nginx/html;
    index index.html index.htm;

    server_name localhost;

    location / {
        try_files $uri $uri/ = 404;
    }
}

I changed the server_name in that block to streamfeed.com rather than localhost, and then added the other block I pasted above. Not sure what the ipv6only=on line is about, since we’re not using ipv6 as far as I know.

Thanks very much for your help. I assume I’m missing something obvious here.