title: First row occurrence of each value
body: <p>I have two varaibles a and amount sorted by a</p>
  
  <pre><code>a      amount
  
  112    12000 
  112    15000 
  113    14000
  114    18000
  114    17000 
  115    19000 
  115    17000
  </code></pre>
  
  <p>I want the  first row occurrence of each value in a variable</p>
  
  <pre><code>output 
  
   a    amount
  112  12000
  113  14000
  114  18000
  115  19000 
  </code></pre>