I experienced this error with my WordPress site but I saw that there were two indexes showing in my developer tools sources.
Chrome Developer Tool Error So I had the thought that if there are two indexes starting at the first line of code then there's a replication and they're conflicting with each other. So I thought that then perhaps it's my HTML minification from my caching plugin tool.
So I turned off the HTML minify setting and deleted my cache. And poof! It worked!