title: Filter dataframe based on aggregate count value of a column
body: <p>Consider a dataframe like the following:</p>
  
  <pre><code>Key | Value
  A   | 1
  A   | 2
  B   | 2
  C   | 3
  </code></pre>
  
  <p>I want to filter this dataframe based on the condition that only get rows with the key where the key occurs more than once.</p>
  
  <p>So the expected output is </p>
  
  <pre><code>Key | Value
  A   | 1
  A   | 2
  </code></pre>
  
  <p>What's the most succinct way of doing this in R? Looking for a generalized solution where count the count can be > n</p>