avoid double-iteration over engine IDs in get_dict

IPython/parallel/client/asyncresult.py

@@ -195,15 +195,17 b' class AsyncResult(object):'
             results = [results]
         engine_ids = [ md['engine_id'] for md in self._metadata ]
         
-        seen = set()
-        for engine_id in engine_ids:
-            if engine_id in seen:
+        
+        rdict = {}
+        for engine_id, result in zip(engine_ids, results):
+            if engine_id in rdict:
                 raise ValueError("Cannot build dict, %i jobs ran on engine #%i"%(
-                    engine_ids.count(engine_id), engine_id))
+                    engine_ids.count(engine_id), engine_id)
+                )
             else:
-                seen.add(engine_id)
+                rdict[engine_id] = result
 
-        return dict(zip(engine_ids,results))
+        return rdict
 
     @property
     def result(self):