diff --git a/lib/sandbox.js b/lib/sandbox.js
index d91224b..6a59d29 100644
--- a/lib/sandbox.js
+++ b/lib/sandbox.js
@@ -39,6 +39,28 @@ const context = vm.createContext();
 
 let state;
 
+const runningExtensions = new WeakMap();
+function runExtension(sessionContext, key, extCall, callback) {
+  let re = runningExtensions.get(sessionContext);
+  if (!re) {
+    re = {};
+    runningExtensions.set(sessionContext, re);
+  }
+
+  if (re[key]) {
+    re[key].push(callback);
+  }
+  else {
+    re[key] = [callback];
+    extensions.run(extCall, function(err, fault, res) {
+      if (!err && !fault)
+        sessionContext.extensionsCache[key] = res;
+      for (let c of re[key])
+        c(err, fault);
+      delete re[key];
+    });
+  }
+}
 
 class SandboxDate {
   constructor() {
@@ -340,13 +362,12 @@ function run(script, globals, sessionContext, startRevision, maxRevision, callba
   let counter = 3;
   for (let key of Object.keys(_state.extensions)) {
     counter += 2;
-    extensions.run(_state.extensions[key], function(err, fault, res) {
+    runExtension(_state.sessionContext, key, _state.extensions[key], function(err, fault) {
       if (err || fault) {
         if (counter & 1)
           callback(err, fault);
         return counter = 0;
       }
-      _state.sessionContext.extensionsCache[key] = res;
       if ((counter -= 2) === 1)
         if (status === 2)
           return run.apply(null, args);