Keyword Cannibalization
When you bring your site directly into the stiff competition by competing to promote using the same keywords other sites are using it is called keyword cannibalization. In such instances, Google finds it hard to push one of the websites up into the SERPs. Only the site that betters on all aspects wins.
Three Types of Keyword Cannibalization
Internal
International
Subdomain
Most of the time cannibalization is due to the four cases of internal linking mentioned below:
Authority Injection
Poor Internal Linking
Overlapping Theme
Non-Contextual Optimization
Authority injection is too much importance given to a page where more than one internal link is injected. Poor internal linking is improper use of anchor text or a repeat of anchor text. In the case of overlapping themes the subject matter overlaps. In the case of non-contextual optimization, the subject matter does not gel with the internal link.
Keyword Saturation
In earlier times keyword stuffing was common and this worked. But, the search engines were quick to sort out this spam. To prevent this spam they updated their algorithms and now about 4% of keyword presence is acceptable. More than 4% of keyword presence is usually taken as spam. Hence webmasters should note this fact and maintain the keyword limit. They should instead resort to semantics which are related to search terms and carry forward the contextual meaning.
If these practices are not rectified or followed properly it would be difficult to rank those pages high on the SERPs.