When two of your pages chase the same query, Google picks one and divides what should have been a single page’s strength between them. The intuition that more pages on a topic is always better runs straight into this; the two pages do not stack into a stronger presence, they split it. Coverage gets diluted rather than doubled, and both versions tend to underperform what one consolidated page would have done.
The mechanism is straightforward once you see what Google is doing. Faced with two pages that answer the same intent, it selects one to rank, and which one it shows can shift over time, swapping between them as it reassesses. Meanwhile the signals that should have concentrated on a single URL, the links, the relevance, the engagement, are spread across both. Neither page accumulates the full weight, so each ranks lower than the combined version would, and you end up competing against yourself for the position you wanted. This is the shape of keyword cannibalization: two pages drawing from the same well, leaving both shallow.
The cost is easy to miss because nothing looks broken. Both pages exist, both get indexed, both even rank for something. But the query you actually care about is being served by a page that carries half the authority it could, while the other half sits on a near-duplicate doing the same job worse. The fix is to stop the split, either by merging the two into one page that absorbs both, or by differentiating them clearly enough that they target genuinely separate intents.
So the editor consolidates or differentiates the two pages instead of leaving them to compete, concentrating the signals on one strong page rather than letting them divide across two weaker ones.