Content cannibalization occurs when various URLs of a given site are visible in Google for the same keywords.
Reasons behind keyword cannibalization:
Similar pages within a single website – e.g. there’re two very similar articles on the same website
Errors in the information architecture – e.g. the same URL is provided with a different number of external links every day
Using the same anchor text to source links for two different URLs
Repetitions in the information architecture – e.g. a store category and an article posted on the store’s blog referring to the same keyword
Other errors in the information architecture (similar products in the online store, misuse of categories and other taxonomies like tags)
Consequences of keyword cannibalization:
Decreased traffic from the search engine
Volatile positions in search results
In the Cannibalization report you will find a summary of the url's in your domain are affected by cannibalization.
Here you will find information on:
the average monthly number of searches for the keyword for which the content is cannibalized
position on which the given url is currently ranked
visibility history of a given url
CPC
url
url which is displayed alternatively
keyword difficulty
Go to the Sections tab to see the sections in the domain particularly affected by cannibalization.
How to cope with cannibalization?
The problem of cannibalization can be solved after identifying the reason behind it. The most typical steps you’ll need to take include:
Introducing changes in the metadata of pages that get cannibalized – this process involves optimizing, for a given keyword, a URL which is ranked higher for that particular keyword or which is believed to be more important
Setting redirect 301 to the right page – if cannibalization is a result of page duplicates (e.g. when various colors of a single products are indexed), it might be useful to set Redirect 301 to the right page (meaning the one which is ranked higher or more important)
Establishing a canonical tag to the right page – an alternative to Redirect 301 can be a canonical tag, that is providing the search engine with the original source of content (here, for different colors of a product, this may be a page without the product colors)
Other optimization efforts for a given URL – from time to time, there can be other optimization work to be done, for example adding new content for a selected keyword on a given page or providing additional elements
Improving the external linking profile – when cannibalization is an effect of external linking, one has to check whether or not the links for the cannibalized keyword lead to the wrong URL within the website’s internal structure. If they do, they have to be redirected to the right URL
Modifying the external linking strategy – when links to a website are acquired from external sources and anchor text including a particular keyword directs to more than one URL, the links should also be directed to one true URL.
Be aware that once you fix an error, the search engine will need a few or a dozen or so days to implement positive changes.