title: AssertionError: Index is not unique on dataframe (Entity cust) using Featuretools
body: <p>I have a dataframe as below</p>
  
  <pre><code>customerid                                   term   age
  08a858899538ddb8e015390510b321f0830199897     30     24
  18a858959537a097401537a4e316e25f730196361     60     72
  a8589c253ace09b0153af6ba58f1f313019822366     45     38
  </code></pre>
  
  <p>I am creating an entity as below using <strong>featuretools</strong></p>
  
  <pre><code>es = es.entity_from_dataframe(entity_id = 'cust', dataframe = df, index = 'customerid')
  </code></pre>
  
  <p>but i get the error</p>
  
  <pre><code>AssertionError: Index is not unique on dataframe (Entity cust)
  </code></pre>
  
  <p>yet customerid is the identifier </p>