Why does my Unity UI Button fire onClick twice?

You have two EventSystem objects in the scene, or one EventSystem with both StandaloneInputModule and InputSystemUIInputModule active. Each module dispatches independently. Delete duplicate EventSystems and remove the legacy module if using New Input System.

How do I check for multiple EventSystems in a Unity scene?

In the Hierarchy, search for 't:EventSystem'. There should be exactly one across all loaded scenes. If you load scenes additively, watch out for each scene bringing its own EventSystem; mark one scene's as the canonical and delete others on load.

Why does my onClick fire on press and release?

That happens when both Pointer Down and Pointer Click events trigger the same handler. UnityEvent on Button only fires onClick (release). If you wired the same method to Pointer Down too, both fire per tap. Remove one wiring.

Fix: Unity UI EventSystem Double Firing Buttons

Quick answer: A scene with two EventSystem objects (often from additive scene loading) dispatches every UI event twice. Remove duplicates so exactly one EventSystem exists. Also, having both StandaloneInputModule and InputSystemUIInputModule on the same object double-dispatches.

Here is how to fix Unity UI Buttons that fire their onClick handler twice per tap. Players say the start button starts the game and immediately exits to menu. Or quantity buttons increment by 2 per click. The cause is duplicated event dispatch — either two EventSystems in the scene or two input modules attached to one.

The Symptom

Tap a UI Button. The onClick handler runs twice. Looking at debug logs, the same button click logs back-to-back with millisecond-level timing.

What Causes This

Duplicate EventSystem. Each scene Unity creates a default EventSystem when the first UI element is added. Additive scene loading brings extras. Both dispatch events on every input.

Two input modules. Migration from old to new Input System sometimes leaves both StandaloneInputModule and InputSystemUIInputModule active. Each dispatches independently.

Manual handler wiring. Wiring the same method to both Pointer Down and Pointer Click on a Button.

Touch + Mouse on touch devices. The browser/OS may synthesize mouse events from touch; both dispatch through the same handler.

The Fix

Step 1: Find duplicate EventSystems.

// Editor utility: count EventSystems
[UnityEditor.MenuItem("Tools/Find EventSystems")]
static void Find()
{
    var all = Object.FindObjectsByType<UnityEngine.EventSystems.EventSystem>(
        FindObjectsSortMode.None);
    Debug.Log($"Active EventSystems: {all.Length}");
    foreach (var es in all)
        Debug.Log($"  {es.gameObject.scene.name}/{es.name}", es);
}

Run during play. If count > 1, delete extras or implement a singleton check.

Step 2: Singleton enforcement at scene load.

using UnityEngine;
using UnityEngine.EventSystems;

[RequireComponent(typeof(EventSystem))]
public class EventSystemSingleton : MonoBehaviour
{
    void Awake()
    {
        var all = Object.FindObjectsByType<EventSystem>(FindObjectsSortMode.None);
        if (all.Length > 1)
        {
            // keep the first; destroy this one
            if (all[0] != GetComponent<EventSystem>())
                Destroy(gameObject);
        }
    }
}

Step 3: Remove old StandaloneInputModule if using new Input System. On the EventSystem, look at attached components. If you see both Standalone Input Module and Input System UI Input Module, delete the legacy one.

Step 4: Audit handler wiring. Open each Button. In Inspector, expand OnClick and Event Trigger components. The same method should not be wired to both.

Step 5: Disable mouse synthesis on mobile. If you only want Touch on mobile, disable the InputSystemUIInputModule’s mouse fallback or use platform branching:

#if UNITY_EDITOR || UNITY_STANDALONE
    // keep mouse for desktop
#else
    var module = GetComponent<InputSystemUIInputModule>();
    module.cancelAction = null;
#endif

Additive Scene Best Practice

Designate one persistent scene that owns the canonical EventSystem (and Audio Listener, Camera, etc.). Other scenes never include their own. Load order matters: load the persistent scene first, then additive levels.

“One EventSystem per session. One input module per EventSystem. Buttons fire once per tap.”

Related Issues

For UI button non-response, see Button Not Responding. For raycaster blocking, see Raycaster Blocking Clicks.

One EventSystem. One input module. No duplicate Event Trigger entries. Single fire.