title: Equivalent of a Vlookup in R
body: <p>I'm trying to merge/join/vlookup data set two into dataset one by the unique_id.
  Dataset two has many duplicates of the same unique_id and information associated with it. Only one column from dataset two is important the amount_due. I would like to add the amount_due column into dataset one by the correct unique_id, dataset two below.</p>
  
  <p>Data Set One</p>
  
  <pre><code>    unique_id  df1  df2  df3     df4
      1234       1    h    8/4/18  no
      2341       2    nl   8/5/18  yes
      3412       3    sg   8/3/18  no
      4213       4    hi   7/3/18  yes
  </code></pre>
  
  <p>Data Set Two</p>
  
  <pre><code>    unique_id  df1  df2  df3     df4  amount_due  df5
      1234       1    h    8/4/18  no   $100        mcd
      1234       1    h    8/4/18  no   $100        mcd
      1234       1    h    8/4/18  no   $100        mcd
      2341       2    nl   8/5/18  yes  $1          hsn
      3412       3    sg   8/3/18  no   $200        bcbs
      3412       3    sg   8/3/18  no   $200        bcbs
      4213       4    hi   7/3/18  yes  $2.22       r
      4213       4    hi   7/3/18  yes  $2.22       r
  </code></pre>
  
  <p>Desired output below</p>
  
  <pre><code>    unique_id  df1  df2  df3     df4  amount_due
      1234       1    h    8/4/18  no   $100
      2341       2    nl   8/5/18  yes  $1
      3412       3    sg   8/3/18  no   $200
      4213       4    hi   7/3/18  yes  $2.22
  </code></pre>