title: TransactionEncoder generates repeated columns
body: <p>i have to apply apriori algorithme to analyse a data in this format:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>item1</th>
  <th>item 2</th>
  <th>item3</th>
  <th>item4</th>
  <th>item5</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>AD_0.5_1</td>
  <td>None</td>
  <td>None</td>
  <td>None</td>
  <td>None</td>
  </tr>
  <tr>
  <td>AD_0.5_1</td>
  <td><strong>AD_0_0.5</strong></td>
  <td>None</td>
  <td>None</td>
  <td>None</td>
  </tr>
  <tr>
  <td><strong>AD_0_0.5</strong></td>
  <td>courbe_gauche</td>
  <td><strong>AG_niveau</strong></td>
  <td><strong>mouille</strong></td>
  <td>None</td>
  </tr>
  <tr>
  <td>AD_0_0.5</td>
  <td>AD_1.5_4</td>
  <td>AG_niveau</td>
  <td>en_s</td>
  <td><strong>mouille</strong></td>
  </tr>
  <tr>
  <td>AD_0_0.5</td>
  <td>AD_1.5_4</td>
  <td>CHA_2_5</td>
  <td>None</td>
  <td>None</td>
  </tr>
  <tr>
  <td>AD_0_0.5</td>
  <td>AD_1.5_4</td>
  <td>CHA_2_5</td>
  <td>None</td>
  <td>None</td>
  </tr>
  <tr>
  <td>AG_denevle</td>
  <td>AD_1.5_4</td>
  <td>CHA_2_5</td>
  <td>plat</td>
  <td><strong>AG_niveau</strong></td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>There no order of items, example items in Bold character in Table, EX: <strong>mouille</strong> is in item4 in the third row but it is in item5 in fourth row.
  There no duplicate item in same row.
  So I apply TransactionEncoder to prepare data for apriori as follows:</p>
  <pre><code>records = []
  for i in range (0, df.shape[0]):
      records.append([str(df.values[i,j]) for j in range(0, df.shape[1])])
  
  TE = TransactionEncoder()
  array = TE.fit(records).transform(records)
  df1 = pd.DataFrame(array, columns = TE.columns_)
  </code></pre>
  <p>then I got multiple repetitions of the same item (in Bold Characters):</p>
  <pre><code>df1.columns
  </code></pre>
  <blockquote>
  <p>Index(['  AD_0.5_1', '  AD_0_0.5', '  AD_1.5_4', '  AD_denivle',
  '  AD_niveau_chaussee', '  AG_denivle', '  AG_niveau_chaussee',
  '  mauvaise', <strong>'mouille'</strong>, <strong>'unique'</strong>, 'AD_0.5_1', 'AD_0_0.5',
  'AD_denivle' <strong>'mouille'</strong>, <strong>'unique'</strong>],
  dtype='object')</p>
  </blockquote>
  <p>so can I generate data without repetitions?</p>