title: Adding and removing existing edge in a graph (BOOST)?
body: <p>Lets say I have an undirected graph G. Lets say I add the following</p>
  
  <pre><code>add_edge(1,2,G);
  add_edge(1,3,G);
  add_edge(0,2,G);
  </code></pre>
  
  <p>Now I lets say add this again:</p>
  
  <pre><code>add_edge(0,2,G);
  </code></pre>
  
  <p>Do I have two edges in my graph from 0 ---> 2 ?</p>
  
  <p>What happens if I added the edge twice and I do:</p>
  
  <pre><code>remove_edge(0,2,G);
  </code></pre>
  
  <p>Do both the edges disappear,or do I still have one of them?</p>