Keyowrd Cannibalisation is an issue that arises when multiple pages are competing for the same keyword to get a good rank in the search engines.This is true but the actual fact is that when 2 or more pages are targeting for the same keyword the search engines will display the best page it thinks and will leave the other pages targetted for the same keyword.The page displayed by the search engines may not be the actual page for the targetted keyword and this will lead to a high bounce back from the users as the page being displalyed mismatched the query string provided by the users.The overall rank of the website will go down as the internal pages will be competing among themselves for the targetted keyword