title: Why does merge result in more rows than original data?
body: <p>When I <code>merge</code> two data frames, the result has more rows than the original data. </p>
  
  <p>In this instance, the <strong>all</strong> dataframe has <strong>104956 rows</strong>, <strong>koppen</strong> has <strong>3968 rows</strong> and <strong>alltest</strong> dataframe has <strong>130335 rows</strong>. Ordinarily, alltest should have had rows equal to or less than <strong>all</strong>.</p>
  
  <p>Why is this inflation happening? I am not sure if giving the reproducible example would help as it does work in the previous instances I have used it.</p>
  
  <pre><code>alltest &lt;- merge(all, koppen, by = "fips", sort = F)
  </code></pre>