Make sure a vertex can't be added twice to a graph

src/data-structures/graph/Graph.js

@@ -13,7 +13,13 @@ export default class Graph {
    * @returns {Graph}
    */
   addVertex(newVertex) {
-    this.vertices[newVertex.getKey()] = newVertex;
+    const key = newVertex.getKey();
+
+    if (this.vertices[key]) {
+      throw new Error('Vertex has already been added before');
+    }
+
+    this.vertices[key] = newVertex;
 
     return this;
   }

src/data-structures/graph/__test__/Graph.test.js

@@ -158,6 +158,19 @@ describe('Graph', () => {
     expect(addSameEdgeTwice).toThrow();
   });
 
+  it('should throw an error when trying to add vertex twice', () => {
+    function addSameEdgeTwice() {
+      const graph = new Graph(true);
+      const vertexA = new GraphVertex('A');
+
+      graph
+        .addVertex(vertexA)
+        .addVertex(vertexA);
+    }
+
+    expect(addSameEdgeTwice).toThrow();
+  });
+
   it('should return the list of all added edges', () => {
     const graph = new Graph(true);