title: R - Combine names with duplicate dates into single row
body: <p>I have a data frame like this:</p>
  
  <pre><code>data.frame(name = c("a", "b", "c", "d"),
             date = c("2018-09", "2018-10", "2018-9", "2018-11"))
  </code></pre>
  
  <p>and I want to combine any of the names that share a date with another into a single row separated by a comma with the second column showing the date they share</p>
  
  <pre><code>data.frame(name = c("a,c", "b", "d"),
             date = c("2018-09", "2018-10", "2018-11"))
  </code></pre>