Merge pull request #24 from DarkLord0206/master

Checked for uniqueness of the label

Author: gdalle

.gitignore

@@ -1,2 +1,3 @@
 Manifest.toml
 docs/build/
+.vscode/settings.json

docs/src/tutorial_graphs.md

@@ -64,6 +64,16 @@ julia> has_vertex(cities, 4)
 false
 ```
 
+Note that we can't add the same city (i.e. vertex label) twice:
+
+```jldoctest graphs
+julia> add_vertex!(cities, :London, "Italy")
+false
+
+julia> nv(cities)
+3
+```
+
 We then check the set of edges:
 
 ```jldoctest graphs

src/graphs.jl

@@ -78,9 +78,12 @@ end
 
 Add a vertex to MetaGraph `g` with label `label` having metadata `data`.
 
-Return true if the vertex has been added, false otherwise.
+Return true if the vertex has been added, false incase the label already exists or vertex was not added.
 """
 function Graphs.add_vertex!(g::MetaGraph, label, data)
+    if haskey(g,label)
+        return false
+    end
     added = add_vertex!(g.graph)
     if added
         v = nv(g)