Harden async's parallel against one function returning multiple times

src/async.js

@@ -39,13 +39,17 @@ function Async() {
 		var threads = arguments;
 		var pending = threads.length;
 		var results = [];
+		var returned = [];
 		return function(f) {
 			var useResult = function(i) {
 				return function(x) {
-					results[i] = x;
-					pending--;
-					if(pending < 1) {
-						f(results);
+					if(!returned[i]) {
+						results[i] = x;
+						returned[i] = true;
+						pending--;
+						if(pending < 1) {
+							f(results);
+						}
 					}
 				};
 			};