Keyword cannibalisation happens when two or more pages on the same site are targeting the same search term, whether that was intentional or not. Instead of one strong page ranking well, Google has to choose between them, and the result is usually that both perform worse than a single, clearly focused page would.
How it actually happens
It's rarely deliberate. A common pattern is a business writing a blog post about a topic, then later building a proper service page covering similar ground, without realising the two are now targeting almost the same search. Another is a site with several near identical location or product pages, each one competing for the same handful of terms instead of owning a distinct one each.
It can also happen gradually as a site grows. Content gets added over time by different people, with nobody keeping track of which page is meant to rank for which term, until eventually several pages are quietly fighting each other for the same spot.
How to spot it
The clearest sign is searching your own target keyword and seeing two of your own pages appear in the results, sometimes swapping positions with each other from one week to the next. That swapping is a strong signal Google itself isn't sure which page deserves to rank, which usually means neither one is performing as well as it could.
Search Console is useful here too. If a single keyword shows impressions split across multiple URLs on your own site, rather than concentrated on one, that's cannibalisation showing up in the data.
Quick check: list your five most important target keywords, then check whether more than one page on your site is genuinely trying to rank for each one. If so, that's worth fixing before writing anything new.
The fix that usually works
The first step is deciding which page should actually own the keyword, usually the one that's stronger, more complete, or more commercially important. From there, the weaker page either gets merged into the stronger one, redirected if it's no longer needed, or rewritten to target a genuinely different, more specific keyword instead.
Internal linking matters here too. Once one page is clearly established as the "main" page for a topic, other related pages should link to it using consistent language, rather than each one trying to rank for the exact same term independently.
The bottom line
Cannibalisation isn't usually caused by bad content, it's caused by too many pages aiming at the same target. Fixing it is rarely about writing something new. It's about deciding, clearly, which page deserves to win, then making sure the rest of the site supports that page instead of quietly competing with it.