I remember asking some time ago whether graph-tool computed BC with parallel
edges and Tiago pointing me to the BGL interface.
The answer is the BGL does take in account parallel edges in the computation
of the number of shortest paths. However, the normalization factor in
graph-tool becomes meaningless when networks have parallel edges as the star
is no longer the benchmark network.
Tiago, would it be possible to include a flag in graph-tool so we can choose
to compute normalized/non-normalized betweeness?