Promises should not be able to be canceled more than once.

src/node.js

@@ -257,6 +257,9 @@ process.Promise.prototype.timeout = function(timeout) {
 };
 
 process.Promise.prototype.cancel = function() {
+  if(this._cancelled) return;
+  this._cancelled = true;
+
   this._events['success'] = [];
   this._events['error'] = [];
 

test/mjsunit/test-promise-cancel.js

@@ -0,0 +1,12 @@
+process.mixin(require('./common'));
+
+var promise = new process.Promise();
+var cancelled = false;
+promise.addCancelback(function(){
+  if(cancelled){
+    assertUnreachable("promise should not cancel more than once");
+  }
+  cancelled = true;
+});
+promise.cancel();
+promise.cancel();