Add an extra sanity check.

OpenRA.Game/Traits/World/Shroud.cs

@@ -113,6 +113,9 @@ namespace OpenRA.Traits
 				explored[c.X, c.Y] = true;
 			}
 
+			if (visibility.ContainsKey(a))
+				throw new InvalidOperationException("Attempting to add duplicate actor visibility");
+
 			visibility[a] = visible;
 			Invalidate();
 		}
@@ -147,6 +150,9 @@ namespace OpenRA.Traits
 			foreach (var c in shrouded)
 				generatedShroudCount[c.X, c.Y]++;
 
+			if (generation.ContainsKey(a))
+				throw new InvalidOperationException("Attempting to add duplicate shroud generation");
+
 			generation[a] = shrouded;
 			Invalidate();
 		}