title: Magento Bug? - Can't subscribe to Newsletter with registered email that is not subscribed
body: <p>2 emails are registered users of my Magento store:</p>
  
  <pre><code>user1@example.com
  user2@example.com
  </code></pre>
  
  <p>When registering, user1@example.com opted in the Newsletter while user2@example.com <strong>did not</strong>.</p>
  
  <p>However when user2@example.com tried to sign up with the Newsletter (the Newsletter sign-up form meant for guests) when logged out, Magento gives an error:</p>
  
  <pre><code>There was a problem with the subscription: This email address is already assigned to another user.
  </code></pre>
  
  <p>Would this be classified as a bug? Because everybody loses in here: the store loses one potential lead, and the customer gets frustrated because it didn't fulfill what he or she wanted.</p>
  
  <p>How can I circumvent this before Magento team solves it?</p>