Harden Async's parallel against one call returning several times

async.js

@@ -39,14 +39,18 @@ function Async() {
 		var threads = arguments;
 		var pending = threads.length;
 		var results = [];
+		var returned = [];
 		return function(f) {
 			var useResult = function(i) {
 				return function(x) {
+					if(!returned[i]) {
 						results[i] = x;
+						returned[i] = true;
 						pending--;
 						if(pending < 1) {
 							f(results);
 						}
+					}
 				};
 			};
 			for(var i = 0; i < threads.length; i++) {