Check label uniqueness before inserting vertex

[gdalle]

Do we do that? I don't think so

[pthpth]

"""
    add_vertex!(g, label, val)

Add a vertex to MetaGraph `g` with label `label` having value `val`.

Return true if the vertex has been added, false incase the label already exists or vertex was not added.
"""
function add_vertex!(g::MetaGraph, label, val)
    if g.haskey(label)
        return false
    added = add_vertex!(g.graph)
    if added
        v = nv(g)
        g.vprops[label] = (v, val)
        g.metaindex[v] = label
    end
    added
end

Changing the addvertex! function to this

[gdalle]

Thanks @DarkLord0206, that looks about right! Do you want to prepare a pull request with this change and the relevant tests?

[pthpth]

Yes. I will do the same.

[gdalle]

Nice! If you need help feel free to ping me

[pthpth]

#24 is the PR for the following issue. Please accept it.