diff --git a/lib/mosaik/graph/vertex.rb b/lib/mosaik/graph/vertex.rb
index ab7eb89..cb351ba 100644
--- a/lib/mosaik/graph/vertex.rb
+++ b/lib/mosaik/graph/vertex.rb
@@ -10,12 +10,12 @@ class Vertex
 
       def initialize(id, attributes = {})
         @id = id
-        @edges = {}
+        @edges = Hash.new { |h, k| h[k] = [] }
         @attributes = attributes
       end
 
       def add_edge(to, **attributes)
-        edges[to] = Edge.new(attributes)
+        edges[to] << Edge.new(attributes)
       end
 
       def remove_edge(id)
diff --git a/spec/mosaik/graph/graph_spec.rb b/spec/mosaik/graph/graph_spec.rb
index df33ff1..327ee52 100644
--- a/spec/mosaik/graph/graph_spec.rb
+++ b/spec/mosaik/graph/graph_spec.rb
@@ -32,16 +32,17 @@
       graph.add_directed_edge("vertex1", "vertex2", key: "value")
 
       expect(graph.find_vertex("vertex1").edges.keys).to eq ["vertex2"]
-      expect(graph.find_vertex("vertex1").edges["vertex2"].attributes).to eq key: "value"
+      expect(graph.find_vertex("vertex1").edges["vertex2"].first.attributes).to eq key: "value"
     end
 
-    it "does not add a directed edge twice" do
+    it "adds a directed edge twice" do
       graph.add_vertex("vertex1")
       graph.add_vertex("vertex2")
       graph.add_directed_edge("vertex1", "vertex2")
       graph.add_directed_edge("vertex1", "vertex2")
 
       expect(graph.find_vertex("vertex1").edges.keys).to eq ["vertex2"]
+      expect(graph.find_vertex("vertex1").edges["vertex2"].size).to eq 2
     end
   end
 
@@ -60,16 +61,17 @@
       graph.add_undirected_edge("vertex1", "vertex2", key: "value")
 
       expect(graph.find_vertex("vertex1").edges.keys).to eq ["vertex2"]
-      expect(graph.find_vertex("vertex1").edges["vertex2"].attributes).to eq key: "value"
+      expect(graph.find_vertex("vertex1").edges["vertex2"].first.attributes).to eq key: "value"
     end
 
-    it "does not add an undirected edge twice" do
+    it "adds an undirected edge twice" do
       graph.add_vertex("vertex1")
       graph.add_vertex("vertex2")
       graph.add_undirected_edge("vertex1", "vertex2")
       graph.add_undirected_edge("vertex1", "vertex2")
 
       expect(graph.find_vertex("vertex1").edges.keys).to eq ["vertex2"]
+      expect(graph.find_vertex("vertex1").edges["vertex2"].size).to eq 2
     end
   end
 
diff --git a/spec/mosaik/graph/vertex_spec.rb b/spec/mosaik/graph/vertex_spec.rb
index b716812..c9552bd 100644
--- a/spec/mosaik/graph/vertex_spec.rb
+++ b/spec/mosaik/graph/vertex_spec.rb
@@ -20,14 +20,15 @@
       vertex.add_edge("child", key: "value")
 
       expect(vertex.edges.keys).to eq ["child"]
-      expect(vertex.edges["child"].attributes).to eq key: "value"
+      expect(vertex.edges["child"].first.attributes).to eq key: "value"
     end
 
-    it "does not add an edge twice" do
+    it "adds an edge twice" do
       vertex.add_edge("child")
       vertex.add_edge("child")
 
       expect(vertex.edges.keys).to eq ["child"]
+      expect(vertex.edges["child"].size).to eq 2
     end
   end
 end