This means there are 2 index/page files for your website. We recommend deleting one since your server will randomly choose one over the other, and that means 2 different versions of your site can be shown over the other.
The 2 recommended course of actions are:
a) Republish all files by going to File > Republish All Files
b) Log onto your server and delete the oldest index file
Key to this is that RW publishing is, in a sense, non-destructive. So if you publish a page, then later change the page/folder name and republish - both the old and the new will be on the server. Not a big deal for folders but it is for index files.
The issue you have normally happens because you have an html page and at some stage add something that needs php such as a contact form or widget. The page index will automatically be changed from .html to .php but because publishing is non destructive, you end up with both index pages.