<p>Usually I comment, and ask for clarification what part of the question is not already answered by Wikipedia. Then I vote to close as "unclear what you are asking" or "too broad".</p>

<blockquote>
  <p><strong>unclear what you're asking</strong> Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking.</p>
</blockquote>

<p>If Wikipedia has a nice answer already, people should read that first, and then clarify where they did not understand Wikipedia.</p>

<blockquote>
  <p><strong>too broad</strong>: There are either too many possible answers, or <strong>good answers would be too long for this format</strong>. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.</p>
</blockquote>

<p>Often good Wikipedia articles <em>are</em> "too long for this format".</p>

<p>Here is an example of such a question that IMHO is "too broad" (and answered by Wikipedia):</p>

<p><a href="https://stats.stackexchange.com/questions/223875/can-anyone-help-me-to-find-out-the-working-of-expectation-maximisation-clusterin">https://stats.stackexchange.com/questions/223875/can-anyone-help-me-to-find-out-the-working-of-expectation-maximisation-clusterin</a></p>

<p>Usually I consider <em>closing</em> such questions to be appropriate. If they are well answered by Wikipedia, they are usually not well posed questions. If we don't close them, people will just waste their time on adding yet another slightly different answer.</p>

<p>In some cases, I also post an answer that cites <em>only the relevant paragraph</em> from Wikipedia with an prominent link to hint that they should have read that page... I doubt that Wikipedia "changing" is a major problem for us; and users <em>can</em> go back in time with Wikipedia. Answers have time information that is accurate enough to find the Wikipedia page at that time, usually. I doubt that for this kind of questions, Wikipedia ever will change to a substantially worse article. If this ever happens, only one person needs to investigate this, and edit the question with the permanent revision URL. But in my opinion, we have more incorrect answers on SE than incorrect information in Wikipedia...</p>
