title: MongoDB duplicate value under same collection but different sub collection
body: <pre><code>{
      col1:{
          key1: "val1"
      },
      col2: {
          key1: "val1"            
      }        
  }
  </code></pre>
  
  <p>If I have these 2 small collections within the large collection, how does it work to be <strong>allowed</strong> to have "val1" in col1 and col2, but not:</p>
  
  <pre><code>{
      col1:{
          key1: "val1",
          key2: "val1"
      },
      col2: {
          key1: "val1"
          key2: "val1"
      }        
  }
  </code></pre>