title: Join columns without unique keys and write down non-unique combinations in a new column
body: <p>I have two dataframes:</p>
  <p>DataframeA:</p>
  <pre><code>Gene Bla Bla
  AAA  xxx xxx
  ABA  xxx xxx
  ABA  xxx xxx
  ABC  xxx xxx
  </code></pre>
  <p>Dataframe 2; ID field is unique, Gene is not.</p>
  <pre><code>ID Gene
  123 AAA
  124 AAA
  125 ABA
  126 ABC
  </code></pre>
  <p>What I want: keep the &quot;Gene&quot; column of the first dataframe intact, but add a new column with IDs that corresponds to the same gene</p>
  <pre><code>Gene Bla1 Bla2 ID
  
  AAA  xxx xxx 123&amp;124 
  AAA  xxx xxx 123&amp;124
  ABA  xxx xxx 125
  ABA  xxx xxx 125 
  ABC  xxx xxx 126
      
  </code></pre>
  <p>Many thanks in advance!</p>