diff --git a/client/src/templates/Challenges/utils/worker-executor.js b/client/src/templates/Challenges/utils/worker-executor.js
index 797a0e55cf3..25e4dfb60b9 100644
--- a/client/src/templates/Challenges/utils/worker-executor.js
+++ b/client/src/templates/Challenges/utils/worker-executor.js
@@ -129,16 +129,17 @@ const eventify = self => {
 
   self.emit = (event, ...args) => {
     if (typeof self._events[event] !== 'undefined') {
-      self._events[event].forEach(listener => {
+      const listeners = self._events[event].slice();
+      for (let listener of listeners) {
         listener.apply(self, args);
-      });
+      }
     }
     return self;
   };
 
   self.once = (event, listener) => {
     self.on(event, function handler(...args) {
-      self.removeListener(handler);
+      self.removeListener(event, handler);
       listener.apply(self, args);
     });
     return self;
diff --git a/client/src/templates/Challenges/utils/worker-executor.test.js b/client/src/templates/Challenges/utils/worker-executor.test.js
index 6e44c600bb8..2ebec5833fc 100644
--- a/client/src/templates/Challenges/utils/worker-executor.test.js
+++ b/client/src/templates/Challenges/utils/worker-executor.test.js
@@ -232,3 +232,15 @@ it('Worker executor should get worker from specified location', async () => {
   expect(global.Worker).toBeCalledTimes(1);
   expect(global.Worker).toBeCalledWith('/other/location/test.js');
 });
+
+it('Task should only emit handler once', () => {
+  mockWorker();
+  const testWorker = createWorker('test');
+  const task = testWorker.execute('test');
+  const handler = jest.fn();
+  task.once('testOnce', handler);
+
+  task.emit('testOnce', handler);
+  task.emit('testOnce', handler);
+  expect(handler).toBeCalledTimes(1);
+});