title: How to raise error if duplicates keys in dictionary
body: <p>I try to raise an error if the user enter a duplicate key in a dictionary.  The dictionary is in a file and the user can edit the file manually.</p>
  
  <p>Example:</p>
  
  <pre><code>dico= {'root':{
                  'a':{'some_key':'value',...},
                  'b':{'some_key':'value',...},
                  'c':{'some_key':'value',...},
                  ...
  
                  'a':{'some_key':'value',...},
                }
        }
  </code></pre>
  
  <p>the new key 'a' already exist...</p>
  
  <p>How can I test dico and warn the user when I load dico from the file? </p>