1 files changed, 13 insertions, 4 deletions

diff --git a/lib/async.js b/lib/async.js
index bbbd05c..09f947f 100644
--- a/lib/async.js
+++ b/lib/async.js
@@ -12,6 +12,15 @@
         return async;
     };
 
+    function only_once(fn) {
+        var called = false;
+        return function() {
+            if (called) throw new Error("Callback was already called.");
+            called = true;
+            fn.apply(root, arguments);
+        }
+    }
+
     //// cross-browser compatiblity functions ////
 
     var _forEach = function (arr, iterator) {
@@ -76,7 +85,7 @@
         }
         var completed = 0;
         _forEach(arr, function (x) {
-            iterator(x, function (err) {
+            iterator(x, only_once(function (err) {
                 if (err) {
                     callback(err);
                     callback = function () {};
@@ -87,7 +96,7 @@
                         callback(null);
                     }
                 }
-            });
+            }));
         });
     };
 
@@ -598,14 +607,14 @@
                     var task = q.tasks.shift();
                     if(q.empty && q.tasks.length == 0) q.empty();
                     workers += 1;
-                    worker(task.data, function () {
+                    worker(task.data, only_once(function() {
                         workers -= 1;
                         if (task.callback) {
                             task.callback.apply(task, arguments);
                         }
                         if(q.drain && q.tasks.length + workers == 0) q.drain();
                         q.process();
-                    });
+                    }));
                 }
             },
             length: function () {