title: How to use `values_fn = {summary_fun}` to summarise duplicates in dplyr
body: <p>When I use dplyr's <code>pivot_wider</code> and there are not uniquely identified rows, I get the following error:</p>
  
  <pre><code>Values are not uniquely identified; output will contain list-cols.
  * Use `values_fn = list` to suppress this warning.
  * Use `values_fn = length` to identify where the duplicates arise
  * Use `values_fn = {summary_fun}` to summarise duplicates
  </code></pre>
  
  <p>I'm interested in the summary of duplicates, but when I set <code>values_fn = summary_fun</code> or <code>values_fun = {summary_fun}</code> it throws an error.  How do I actually summarize duplicates in the way intended by the values_fn parameter?</p>