title: Pandas Concat increases number of rows
body: <p>I'm concatenating two dataframes, so I want to one dataframe is located to another.
  But first I did some transformation to initial dataframe:</p>
  
  <pre><code>scaler = MinMaxScaler() 
  real_data = pd.DataFrame(scaler.fit_transform(df[real_columns]), columns = real_columns)
  </code></pre>
  
  <p>And then concatenate:</p>
  
  <pre><code>categorial_data  = pd.get_dummies(df[categor_columns], prefix_sep= '__')
  train = pd.concat([real_data, categorial_data], axis=1, ignore_index=True)
  </code></pre>
  
  <p>I dont know why, but number of rows increased:</p>
  
  <pre><code>print(df.shape, real_data.shape, categorial_data.shape, train.shape)
  (1700645, 23) (1700645, 16) (1700645, 130) (1703915, 146)
  </code></pre>
  
  <p>What happened and how fix the problem?</p>
  
  <p>As you can see number of columns for train equals to sum of columns real_data and categorial_data</p>