A page adds something when it brings information gain the search results do not already have: original data, a sharper synthesis, a missing angle, first-hand experience, or a clearer decision framework. It repeats the results when it is a paraphrase of what already ranks. The pivot is whether the page brings what the results lack, not whether it covers the topic. Gain is new, not more, and a page that only restates the existing answers in fresh wording has added nothing, however thorough it reads.
The reason “cover the topic more completely” fails as a definition is that more-of-the-same is not gain. If the top results already explain the topic, writing a longer version of the same explanation gives the searcher and the search engine nothing they did not have. Completeness measured against the existing results is just a thorough copy. Gain is measured against the gap: the thing the current results do not supply, which your page does. The question is not “did I cover it” but “what does this page have that none of the others do.”
That gap can take several genuine forms. Original data you gathered or measured is gain, because it did not exist in the results before. A synthesis that connects scattered points into a clearer model is gain, because the insight is new even if the inputs were not. A missing angle the existing pages all skipped, first-hand experience none of them have, or a decision framework that turns a pile of facts into a usable choice, each brings something the results lack. The common thread is that a searcher who has already read the top pages still learns something from yours.
So before you write, name the one thing this page will add that the search results do not already contain. Write it down, the original data, the sharper synthesis, the missing angle, the first-hand experience, or the clearer framework, and confirm it is genuinely absent from the current results. If you cannot name that one thing, you are about to repeat the results, and the page is not yet worth writing. Find the gain first, then write to deliver it.