title: Xcode: "Revoke certificate - Your account already has a signing certificate for this machine but it is not present in your keychain."
body: <p>I'm posting this as a question and providing the answer for the benefit of others running into the same situation and searching for the solution as this had me scratching my head and wasting time and searching past questions until I eventually discovered the cause.</p>
  
  <p>I got this error but nothing about my project; my accounts; my certificates; my mac set up had changed. In particular the certificates are still valid and have not been revoked.</p>
  
  <blockquote>
    <p>Revoke certificate Your account already has a signing certificate for
    this machine but it is not present in your keychain. To create a new
    one, you must first revoke the existing certificate.</p>
  </blockquote>
  
  <p>With no obvious cause for the problem suddenly appearing, I eventually discovered the cause, posted as the answer below.</p>