diff --git a/src/core/singletons/subscriptions.ts b/src/core/singletons/subscriptions.ts
index 3ea609a32..e65b862d5 100644
--- a/src/core/singletons/subscriptions.ts
+++ b/src/core/singletons/subscriptions.ts
@@ -13,7 +13,7 @@
 // limitations under the License.
 
 import { EventEmitter } from '@angular/core';
-import { Observable } from 'rxjs';
+import { Observable, Subscription } from 'rxjs';
 
 /**
  * Subscribable object.
@@ -33,20 +33,29 @@ export class CoreSubscriptions {
      * @param onError Callback to run when the an error happens.
      */
     static once<T>(subscribable: Subscribable<T>, onSuccess: (value: T) => unknown, onError?: (error: unknown) => unknown): void {
-        const subscription = subscribable.subscribe(
+        let unsubscribe = false;
+        let subscription: Subscription | null = null;
+
+        subscription = subscribable.subscribe(
             value => {
-                // Unsubscribe using a timeout because we can receive a value immediately.
-                setTimeout(() => subscription.unsubscribe(), 0);
+                // Subscription variable might not be set because we can receive a value immediately.
+                unsubscribe = true;
+                subscription?.unsubscribe();
 
                 onSuccess(value);
             },
             error => {
-                // Unsubscribe using a timeout because we can receive a value immediately.
-                setTimeout(() => subscription.unsubscribe(), 0);
+                // Subscription variable might not be set because we can receive a value immediately.
+                unsubscribe = true;
+                subscription?.unsubscribe();
 
                 onError && onError(error);
             },
         );
+
+        if (unsubscribe) {
+            subscription.unsubscribe();
+        }
     }
 
 }
diff --git a/src/core/singletons/tests/subscriptions.test.ts b/src/core/singletons/tests/subscriptions.test.ts
new file mode 100644
index 000000000..7c7a08412
--- /dev/null
+++ b/src/core/singletons/tests/subscriptions.test.ts
@@ -0,0 +1,62 @@
+// (C) Copyright 2015 Moodle Pty Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import { CoreSubscriptions } from '@singletons/subscriptions';
+import { BehaviorSubject, Subject } from 'rxjs';
+
+describe('CoreSubscriptions singleton', () => {
+
+    it('calls callbacks only once', async () => {
+        // Test call success function.
+        let subject = new Subject();
+        let success = jest.fn();
+        let error = jest.fn();
+        CoreSubscriptions.once(subject, success, error);
+
+        subject.next('foo');
+        expect(success).toHaveBeenCalledTimes(1);
+        expect(success).toHaveBeenCalledWith('foo');
+
+        subject.next('bar');
+        subject.error('foo');
+        expect(success).toHaveBeenCalledTimes(1);
+        expect(error).not.toHaveBeenCalled();
+
+        // Test call error function.
+        subject = new Subject(); // Create a new Subject because the previous one already has an error.
+        success = jest.fn();
+        CoreSubscriptions.once(subject, success, error);
+
+        subject.error('foo');
+        expect(error).toHaveBeenCalledWith('foo');
+
+        subject.next('foo');
+        subject.error('bar');
+        expect(error).toHaveBeenCalledTimes(1);
+        expect(success).not.toHaveBeenCalled();
+
+        // Test with behaviour subject (success callback called immediately).
+        const beaviourSubject = new BehaviorSubject('foo');
+        error = jest.fn();
+        CoreSubscriptions.once(beaviourSubject, success, error);
+
+        expect(success).toHaveBeenCalledWith('foo');
+
+        beaviourSubject.next('bar');
+        beaviourSubject.error('foo');
+        expect(success).toHaveBeenCalledTimes(1);
+        expect(error).not.toHaveBeenCalled();
+    });
+
+});