title: ValueError: cannot reindex from a duplicate axis pd.concat
body: <p>i am trying to concatenate a pandas dataframe:</p>
  
  <pre><code>def extract_articles(data, article_numbers):
    result = pd.concat(
       [
          data[data['ARTICLENO'] == article_no]['QUANTITY']
          for article_no in article_numbers
       ],
       axis=1,
    ).fillna(0)
    result.columns = article_numbers
    return result
  </code></pre>
  
  <p>When reading more lines (about 100k) from the csv i get the following error:
  ValueError: cannot reindex from a duplicate axis</p>
  
  <p>Here is how my csv basically looks like:</p>
  
  <pre><code>Date,       ArticleNo, Quantity
  2018-07-15, 1005,      150
  2018-07-14, 1005,      165
  2018-07-12, 1005,      160
  2018-07-14, 1008,      230
  2018-07-12, 1008,      245
  </code></pre>
  
  <p>The file is sorted by article number and date. For each date it is possible to have multiple 'article number-quantity-tuples' There can be gaps where some of the article numbers have no data, they are 0.
  Why do i get this error?</p>