I will be creating a resources page with a massive set of links to the best tools. Please do subscribe to get the alert when I post it. I’ll also put up my live audit series shortly on my new Youtube Channel. That has a ton of stuff in it! There’s some neat look over the shoulder audits. A lot of the stuff has to be done manually and by just using a handful of tools. You don’t need too many weapons!
External duplicate content is when you copy-paste content from other websites. Usually without placing a link to the original source or using canonical attribute. Google dislikes such content for one simple reason. It doesn’t add any value to end users and the Internet in general. If you have to choose between copying articles from other sources or not having any at all, I’d suggest you choose the latter.
Of all the tools listed in this article, Moz Link explorer is an old one & quite popular.  If you want to compare backlinks between two or more domains, Open Site Explorer is worth trying. This tool works best when you have a paid account of SEOMOZ though a free version of this tool is good enough to get you started checking the backlinks of your site and the sites of your competitors.