title: How to resolve a "Can only install one of:" conflict?
body: <p>I have installed one package via Composer and it installed Guzzlehttp too, because of the package.
  After that I tried to install another package via Composer, which requires Guzzlehttp too and Composer, tried to install it again.</p>
  <p>But I get that error :</p>
  <blockquote>
  <p>Problem 1</p>
  <ul>
  <li><p>Can only install one of: guzzlehttp/guzzle[6.2.0, 6.0.2].</p>
  </li>
  <li><p>Can only install one of: guzzlehttp/guzzle[6.0.2, 6.2.0].</p>
  </li>
  <li><p>Can only install one of: guzzlehttp/guzzle[6.0.2, 6.2.0].</p>
  </li>
  </ul>
  </blockquote>
  <p>I see what is the problem, but I do not know how to fix it.</p>