title: Merge with duplicates values in R
body: <p>I have two dataframes</p>
  
  <p>db1 like :</p>
  
  <pre><code>date.prix;var1;var2
  2012-10-02;pluf;plof
  2012-12-11;pam;pim
  2013-05-17;plop;plip
  ...
  </code></pre>
  
  <p>db2 like :</p>
  
  <pre><code>date.de.cotation;var3;var4
  2012-10-02;tutu;toto
  2012-10-02;ting;tong
  2013-05-17;gui;guou
  ...
  </code></pre>
  
  <p>The join is date.prix = date.de.cotation</p>
  
  <p>And I want something like :</p>
  
  <pre><code>date.prix;var1;var2;var3;var4
  2012-10-02;pluf;plof;tutu;toto
  2012-12-11;pam;pim;NA;NA
  2013-05-17;plop;plip;gui;guou
  </code></pre>
  
  <p>So : </p>
  
  <ul>
  <li>if there are duplicates in db2, I want values of the first</li>
  <li>if there is no value for the date in db2, I want NAs</li>
  </ul>