title: Add ID column by group
body: <p>I want to create an unique ID in R based on two columns of latitude and longitude so that duplicated  locations have the same cluster ID.</p>
  
  <p>For example:</p>
  
  <pre><code>LAT        LONG    Cluster_ID
  13.5330 -15.4180   1
  13.5330 -15.4180   1
  13.5330 -15.4180   1
  13.5330 -15.4180   1
  13.5330 -15.4170   2
  13.5330 -15.4170   2
  13.5330 -15.4170   2
  13.5340 -14.9350   3
  13.5340 -14.9350   3
  13.5340 -15.9170   4
  13.3670 -14.6190   5
  </code></pre>