When your own pages compete, everyone loses
Authority isn't infinite. Every page that ranks for '18mm plywood' is holding a slice of the trust and links you've earned for that term. Spread that across four half-strong pages and you get four pages stuck on page two, instead of one page on page one.
The fix isn't to delete the extras. It's to decide which page should win, then feed the others' strength into it. Google gets one clear answer, and your best page climbs.
Find it with one Google search
Go to Google and search your own domain, a space, then the keyword in quotes, like this: youryard.co.uk "18mm plywood". That tells Google to show only pages on your site that are relevant to that exact phrase.
Now look at what comes back. If several of your own pages show up for the same keyword, that's your cannibalisation. Note them down: one of these is going to be the page you back, the rest are going to point at it.
Pick one target page, point the rest at it
Choose the page you most want to rank: usually the one that can actually take an order, so a category or product page over an old blog post. That's your target.
Then go into each of the other ranking pages and add an internal link to the target, using the keyword as the link text. This passes the authority those pages have already earned into the one page you want to win. You're not deleting anything, you're pooling your strength behind a single answer.
No good page for the keyword? Build one.
Sometimes the search shows you're ranking for a keyword with nothing but scraps: a stray mention on a page that isn't really about it. In that case, build the page it deserves.
Put the keyword in the four spots that matter most: the page title, the URL slug, the H1, and the first sentence of the copy. Then link your existing ranking pages into that new page, exactly as above. You're giving the keyword a proper home and pointing your earned authority straight at it.
This isn't the same as duplicate content
Worth being clear, because they get muddled. Duplicate content is the same text living on two URLs, which you fix with canonicals. Cannibalisation is different pages, different content, all chasing the same keyword and splitting the vote. The fix here is editorial: choose a winner and consolidate links into it.
For merchants, the biggest cannibalisation offender is usually a thin blog post outranking the category page you actually want people to land on and buy from. Point the post at the category, and let the money page win.
What to do this week
Take your five most important keywords. Run youryard.co.uk "keyword" for each and see how many of your own pages show up. Where more than one does, pick the page that can take an order as the target.
Then add internal links from the also-rans into that target, using the keyword as the anchor text. It's an afternoon of small edits that stops your own pages from cancelling each other out, and it usually moves the winner up within a few weeks.