What is keyword cannibalization?

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If you optimize your articles for similar terms, your rankings might suffer from keyword cannibalization: you'll be ‘devouring' your own chances to rank in Google! Especially when your site is growing, chances are your content will start competing with itself. Here, I'll explain why keyword cannibalism can be detrimental to SEO, how you can recognize it and what to do about it.
What is keyword cannibalization?

Keyword cannibalization means that you have various blog posts or articles on your site that can rank for the same search query in Google. Either because the topic they cover is too similar or because you optimized them for the same keyphrase. If you optimize posts or articles for similar search queries, they're eating away at each other's chances to rank. Usually, Google will only show 1 or 2 results from the same domain in the search results for a specific query. If you're a high authority domain, you might get 3.
An example of keyword cannibalization

Let's look at an example of optimizing posts for a similar keyphrase. I wrote two posts about whether or not readability is a ranking factor. The post ‘Does readability rank?‘ was optimized for [does readability rank], while the post ‘Readability ranks!‘ was optimized for the focus keyword [readability ranking factor]. The posts had a different angle but were still very similar. For Google, it is hard to figure out which of the two articles is the most important.
Update: Did you see the same article? That's correct, by now we've fixed this cannibalization issue, but we've kept this example for the sake of illustration.
Why is keyword cannibalism bad for SEO?

If you cannibalize your own keywords, you're competing with yourself for ranking in Google. Let's say you have two posts on the exact same topic. In that case, Google can't distinguish which article should rank highest for a certain query. In addition, important factors like backlinks and CTR get diluted over several posts instead of one. As a result, they'll probably both rank lower. Therefore, our SEO analysis will give a red bullet whenever you optimize a post for a focus keyword you've used before.
How to recognize it?

Checking whether or not your site suffers from keyword cannibalism is easy. You simply do a search for your site, for any specific keyword you suspect might have multiple results. In my case, I'll google site:yoast.com readability ranks. The first two results are the articles I suspected to suffer from cannibalization.

Googling ‘site:domain.com “keyword” will give you an easy answer to the question if you're suffering from keyword cannibalism or not. You can check your findings by typing the same keyword into Google (using a private browser or local search result checker like https://valentin.app/). Which of your pages do you see in the search results, and what position do they rank? Of course, if two of your pages for the same keyword are ranking #1 and #2, that's not a problem. But do you see your articles, for example on positions 7 and 8? Then it's time to sort things out!
How do you resolve keyword cannibalization?

We have an extensive article written by Joost that explains how to find and fix cannibalization issues on your site. It clearly describes the four steps you should take to solve these kind issues:

Audit your content
Analyze content performance
Decide which ones to keep
Act: merge, delete, redirect

The first two steps will help you decide which articles to keep and which ones to merge or delete. In many cases, step 4 will consist of combining and deleting articles, but also improving the internal linking on your site.
Merge or combine articles

If two articles attract the same audience and tell the same story, you should combine them. Rewrite the two posts into one amazing, kickass article. That'll help your rankings (Google loves lengthy and well-written content), and solve your keyword cannibalization problem.
In fact, that's exactly what we did with our two posts on readability being a ranking factor. You can use the Yoast Duplicate Post plugin to clone one of the posts and work from there. And don't just press the delete button! Always make sure to redirect the post you delete to the one you keep. If that's something you're struggling with, Yoast SEO Premium can help: It makes creating redirects as easy as pie!
Improve internal linking

You can help Google figure out which article is most important by setting up a decent internal linking structure. This means that you should link from posts that are less important to posts that are the most important to you. That way, Google can figure out (by following links) which ones you want to pop up highest in the search engines.
Your internal linking structure could solve a part of your keyword cannibalism problems. You should think about which article is most important to you and link from the less important long-tail articles, to your most important article. Read more about how to do this in my article about ranking with cornerstone content.
Keyword cannibalization and online shops

Now, if you have an online shop, you might be worried about all those product pages targeting similar keywords. For online shops, it makes sense that there are multiple pages for products that are alike. If that's the case for you, then it's very important to give site structure some thought. A good strategy is to link back from every product page to your category page – the page you should optimize to rank. In addition, you should keep an eye on old product pages that could potentially cannibalize more important pages. If that's the case, simply delete and redirect those. And don't forget that Yoast SEO Premium can help make redirecting easier with its handy redirect manager!
Keyword cannibalism will affect growing websites

If your site gets bigger, your chances will increase that you face keyword cannibalism on your own website. You'll be writing about your favorite subjects and without even knowing it, you'll write articles that end up being rather similar. That's what happened to me too. Once in a while, you should check the keywords you want to rank for the most. Make sure to check whether you're suffering from keyword cannibalism. You'll probably need to make some changes in your site structure or to rewrite some articles every now and then.