title: Instances appearing twice when extending signatures
body: <p>When I execute the following example using Alloy, either v4.2 or v5, I get an instance that appear twice in solution space. </p>
  
  <pre><code>sig A {}
  sig B extends A { }
  pred P { }
  run P for 2
  </code></pre>
  
  <p>Generated instances:</p>
  
  <pre><code>1:A={}, B={}
  2:A={A$0}, B={}
  3:A={B$0}, B={B$0}
  4:A={A$0, B$0}, B={B$0}
  5:A={A$0, A$1}, B={}
  6:A={A$0, B$0}, B={B$0} // same as instance 4
  7:A={B$0, B$1}, B={B$0, B$1}
  </code></pre>
  
  <p>Any suggestions?</p>