title: Concatenate two dataframes and drop duplicates in Pandas
body: <p>For <code>df2</code> which only has data in the year of <code>2019</code>:</p>
  
  <pre><code>  type  year  value
  0    a  2019     13
  1    b  2019      5
  2    c  2019      5
  3    d  2019     20
  </code></pre>
  
  <p><code>df1</code> has multiple years data:</p>
  
  <pre><code>  type  year  value
  0    a  2015     12
  1    a  2016      2
  2    a  2019      3
  3    b  2018     50
  4    b  2019     10
  5    c  2017      1
  6    c  2016      5
  7    c  2019      8
  </code></pre>
  
  <p>I need to concatenate them together while replacing <code>df2</code>'s values in <code>2019</code> with the values from <code>df1</code>'s same year.</p>
  
  <p>The expected result will like this:</p>
  
  <pre><code>  type  date  value
  0    a  2015     12
  1    a  2016      2
  2    b  2018     50
  3    c  2017      1
  4    c  2016      5
  5    a  2019     13
  6    b  2019      5
  7    c  2019      5
  8    d  2019     20
  </code></pre>
  
  <p>The result from <code>pd.concat([df1, df2], ignore_index=True, sort =False)</code>, which clearly have multiple <code>value</code>s in <code>year</code> of 2019 for one <code>type</code>. How should I improve the code? Thank you.</p>
  
  <pre><code>   type  date  value
  0     a  2019     13
  1     b  2019      5
  2     c  2019      5
  3     d  2019     20
  4     a  2015     12
  5     a  2016      2
  6     a  2019      3
  7     b  2018     50
  8     b  2019     10
  9     c  2017      1
  10    c  2016      5
  11    c  2019      8
  </code></pre>