Error on creating directed graph about fsharp.fgl

[LibraChris]

Thank you very much for pointing this out, I updated this error message earlier today and will publish the updated NuGet package later today.
In case you want to use Louvain on the presented graph, I have 2 details to point out to you:

1. For the usage of the Louvain method the FGL.ArrayAdjacencyGraph module is needed, so you would have to use that instead of the standard FGL Graph model. Additionally, while you can use the ArrayAdjacencyGraph Model to get directed Graph info, it is generally an undirected graph model optimized for the Louvain algorithm.
2. The Louvain method is optimized for undirected Graphs since the definition of modularity for directed graphs is debatable. You can treat a directed graph as undirected to perform Louvain, but the original Louvain Algorithm is specific for undirected graphs.

There exist some suggestions on how to treat modularity and therefore Louvain in directed graphs and I am working on upgrading the FGL library to include such a directed Modularity approach in the future, but as of now a specific implementation of a directed Louvain approach for community detection is not implemented in Fsharp.FGL.

[HarryMcCarney]

This is actually caused by trying to create duplicate edges. Id suggest updating the error message to say this.