Nginx duplicate default server error
8 months late, but anyway...
I had the same problem - it turned out that my editor left a default~ backup file behind after I had edited default. Nginx read both the backup and the edited version and of course found a discrepancy between the two. Simply deleting the backup allowed Nginx to start.
In my case I updated both GitLab and the Ubuntu server to 14.04. That apparently made nginx to get a new default config file. Just deleted it keeping the gitlab config made it happy again.