fix bug where multiple messages between same engines caused build crash

encompass-cs/WorldBuilder.cs

@@ -187,7 +187,7 @@ namespace Encompass
         {
             foreach (var senderEngine in senders)
             {
-                foreach (var messageType in senderEngine.sendTypes.Where((type) => type.GetInterfaces().Contains(typeof(IMessage))))
+                foreach (var messageType in senderEngine.sendTypes)
                 {
                     if (typeToReaders.ContainsKey(messageType))
                     {
@@ -195,7 +195,10 @@ namespace Encompass
                         {
                             if (senderEngine != readerEngine)
                             {
-                                engineGraph.AddEdge(senderEngine, readerEngine);
+                                if (!engineGraph.Exists(senderEngine, readerEngine))
+                                {
+                                    engineGraph.AddEdge(senderEngine, readerEngine);
+                                }
                             }
                         }
                     }

test/WorldBuilderTest.cs

@@ -518,5 +518,48 @@ namespace Tests
                 resultMessages.First().Should().BeOfType<AMessage>();
             }
         }
+
+        public class MultipleMessagesBetweenEngines
+        {
+            static List<Engine> order = new List<Engine>();
+
+            struct AMessage : IMessage { }
+            struct BMessage : IMessage { }
+
+            [Sends(typeof(AMessage), typeof(BMessage))]
+            class AEngine : Engine
+            {
+                public override void Update(double dt)
+                {
+                    order.Add(this);
+                }
+            }
+
+            [Receives(typeof(AMessage), typeof(BMessage))]
+            class BEngine : Engine
+            {
+                public override void Update(double dt)
+                {
+                    order.Add(this);
+                }
+            }
+
+            [Test]
+            public void WorldBuilderDoesNotThrowError()
+            {
+                var worldBuilder = new WorldBuilder();
+
+                var engineA = worldBuilder.AddEngine(new AEngine());
+                var engineB = worldBuilder.AddEngine(new BEngine());
+
+                Assert.DoesNotThrow(() => worldBuilder.Build());
+
+                var world = worldBuilder.Build();
+
+                world.Update(0.01f);
+
+                Assert.That(order.IndexOf(engineA), Is.LessThan(order.IndexOf(engineB)));
+            }
+        }
     }
 }