title: Exception: cannot handle a non-unique multi-index
body: <p>I am trying to read a csv, add columns and then set values for each cell in the new columns. but I can the below error</p>
  
  <pre><code> raise Exception("cannot handle a non-unique multi-index!")
  Exception: cannot handle a non-unique multi-index!
  </code></pre>
  
  <p>My csv data does not have any index column, I don't know how add an incremental numeric index column to it.</p>
  
  <p>I tried <code>print("data.columns = ", data.columns)</code> but I can see all of the columns are part of the index which is incorrect.</p>
  
  <pre><code>data = pd.read_csv( filename, names=colnames, header=None )
  # data = data.drop(['datetime'], axis=1)
  data[ "positive" ] = 0
  data[ "negative" ] = 0
  data[ "hate_speech" ] = 0
  data[ "neither" ] = 0
  data[ "count" ] = 0
  data[ "class" ] = -1
  print("data.columns = ", data.columns)
  for j in range(0, data.shape[0]):
      tweet_set = set( str(data[ "tweet" ][ j ]).split() )
  
      pos_count = len(pos_set.intersection( tweet_set ))
      data.at[ j , "positive"] = pos_count
  </code></pre>