Keyword cannibalization

Keyword cannibalization is what happens when two or more pages on the same site are written for the same query, so they compete with each other for the one place the site could have held and the search engine has to choose between them.

How the engine settles it

If the pages are close enough to be duplicates, Google groups them and picks a canonical, and the others become alternates that rarely show. If they are distinct but overlapping, both may be indexed and the signals that would have gathered on one, links among them, are split across two. The site can state its preference with a permanent redirect, a rel="canonical" element or header, or a sitemap entry, in descending order of strength, and Google treats the statement as a hint it may override when another version serves searchers better.

Telling it from coverage

Two pages on one subject serving two different needs are not cannibalising. A page that explains a service and a page that lists its prices share a topic and answer different questions. The test is whether a reader who landed on one would have been equally served by the other; if yes, there is one page’s worth of content on two URLs. The remedies follow from that: merge and redirect when the pages say the same thing, or rewrite one until it answers something the other does not.

In a generated answer

This site has not measured what overlapping pages do to an assistant’s answer. What follows from the mechanism is narrow. An assistant fetches a handful of pages and lifts a passage; two pages holding the same passage give it the same thing twice and nothing more, and neither is more likely to be chosen for being duplicated.

A common question

Do two pages on the same subject always count as keyword cannibalization?

No. Two pages can share a subject and still serve different needs, as a page explaining a service and a page listing its prices do. The test is whether a reader who landed on one would have been equally well served by the other; when the answer is yes, one page's worth of content is spread over two URLs. The remedy follows from that reading: merge and redirect where the pages say the same thing, or rewrite one until it answers something the other does not.

Sources

Google Search Central, how to specify a canonical with rel=canonical and other methods.