title: How to avoid duplicate values from the url with faceted search?
body: <p>Standard feceting example from django-haystack documentation. I use FacetedSearchView and FacetedSearchForm.</p>
  
  <p>Provided links:</p>
  
  <pre><code>age
  20 (4)
  18 (2)
  21 (2)
  22 (2)
  24 (2)
  
  price
  200 (8)
  150 (6)
  100 (2)
  300 (2)
  400 (2)
  </code></pre>
  
  <p>When I chooses a link <code>20 (4)</code>, will narrow the search query to only include those conditions and display the further facets. </p>
  
  <p>Provided URL: <code>?q=+&amp;selected_facets=age_exact:20</code></p>
  
  <p>Provided links:</p>
  
  <pre><code>age
  20 (4)
  18 (0)
  21 (0)
  22 (0)
  24 (0)
  
  price
  150 (2)
  200 (2)
  100 (0)
  300 (0)
  400 (0)
  </code></pre>
  
  <p>Until then, it's ok.</p>
  
  <p>Once again, when I chooses a link <code>20 (4)</code>, will display the same facets but 
  to the URL will add the same value: <code>?q=+&amp;selected_facets=age_exact:20&amp;selected_facets=age_exact:20</code></p>
  
  <p>...and so on 
  <code>?q=+&amp;selected_facets=age_exact:20&amp;selected_facets=age_exact:20&amp;selected_facets=age_exact:24&amp;selected_facets=age_exact:21&amp;selected_facets=age_exact:20&amp;selected_facets=age_exact:18</code></p>
  
  <p>How to avoid adding new <code>&amp;selected_facets=age_exact:</code> instead of changing the current value?</p>