title: combine two arrays and sort
body: <p>Given two sorted arrays like the following:</p>
  
  <pre><code>a = array([1,2,4,5,6,8,9])
  
  b = array([3,4,7,10])
  </code></pre>
  
  <p>I would like the output to be:</p>
  
  <pre><code>c = array([1,2,3,4,5,6,7,8,9,10])
  </code></pre>
  
  <p>or:</p>
  
  <pre><code>c = array([1,2,3,4,4,5,6,7,8,9,10])
  </code></pre>
  
  <p>I'm aware that I can do the following:</p>
  
  <pre><code>c = unique(concatenate((a,b))
  </code></pre>
  
  <p>I'm just wondering if there is a faster way to do it as the arrays I'm dealing with have millions of elements.</p>
  
  <p>Any idea is welcomed. Thanks</p>