what exactly is keyword cannibalization?

Two pages showing for the same query isn't automatically a problem. If one is a how-to post and the other is the service page and both sit on page one, you're taking up more of the results - leave them alone. It's only cannibalisation when they keep swapping places and neither one settles high.

Two of your own pages going after the same query, so Google has to pick one and it does not always pick the one you would. At a small scale it almost always shows up as an old blog post outranking the actual service page. Easiest check is Search Console: filter Performance by the query, then look at the Pages tab. If two or three URLs are trading impressions on it week to week, that is your cannibalisation. The fix is usually folding the weaker page into the stronger one and redirecting it, not rewriting both.