Keyword cannibalisation is two or more of your own pages being shown for the same search, splitting its clicks and links while Google chooses which one to show.
Keyword cannibalisation is when two or more of your pages are shown for the same search. They split the clicks and the links between them, and Google decides which one to show, which may not be the one you would choose.
It usually starts with two articles on nearly the same subject. You can spot it in Search Console: filter the Performance report to one search, open the Pages tab, and see whether more than one page has a real share of its impressions.
The fix is to pick one page to answer the search, merge the other into it, and redirect the old address. Google's advice on consolidating duplicate URLs says the same. Before writing a new page, check none of yours already answers the search; see how to find searches you almost rank for. See also canonical tag.