We no need to add vertex if it exists

[3xh4l3]

If we have already added vertex earlier, no need to add it again. Moreover it breaks drawio graph.

Before

After

[lautriv]

I was just about to open a PR for this exact issue and am bummed I didn't think to check sooner! I've been convinced that all of the layout algorithms in igraph are garbage because it seems like no matter what I did to my DrawIO diagram they never looked good. As another user of this library I would love to see this PR merged.

[dmulyalin]

Hello, was trying to reproduce the problem using from_dict method, but no luck, not seeing nodes added twice. N2G attempts to deduplicate nodes/links when they are loaded into the diagram, by the time igraph layout method called, there are should be no duplicate nodes or links present, so, what are you doing to reproduce the problem? @lautriv @3xh4l3