title: NetworkX MultiDiGraph : Adding new attribute to existing node
body: <p>so I came across this situation when trying out MultiDiGraph and I would appreciate some guidance:</p>
  <pre><code>G = nx.MultiDiGraph()
  setA= ['a','b','c','d','e','f']
  G.add_nodes_from(setA,entity='typeA')
  
  setB = ['a','x','y','z']
  G.add_nodes_from(setB,entity='typeB')
  print([node[0] for node in G.nodes(data='entity') if node[1]=='typeA'])
  </code></pre>
  <p>Apparently, this results in overwriting the 'entity' attribute of node 'a' and printing '['b', 'c', 'd', 'e', 'f']'</p>
  <p>Since I suspect this is intentional
  on part of NetworkX, is there any viable workaround for what I am trying to achieve?</p>
  <p>Thanks a lot~</p>