title: Find strings where the first half matches the second
body: <p>I have a list of IP address pairs separated by &quot;::&quot;.</p>
  <pre><code>ip_pairs &lt;- c(&quot;104.124.199.136::192.168.1.67&quot;, &quot;104.124.199.136::192.168.137.174&quot;, &quot;192.168.1.67::104.124.199.136&quot;, &quot;192.168.137.174::104.124.199.136&quot;)
  </code></pre>
  <p>As you can see, the third and fourth elements of the vector are the same as the first two, but reversed (my actual problem is to find all unique pairings of IPs, so the solution would drop the pair B::A if A::B is already present. This could be solved using stringr or regex, I'm guessing.</p>