diff --git a/CHANGELOG-PRERELEASE.md b/CHANGELOG-PRERELEASE.md
index 22142b83d..3fc02af30 100644
--- a/CHANGELOG-PRERELEASE.md
+++ b/CHANGELOG-PRERELEASE.md
@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog].
 
 ### Fixed
 - Apply On Play may not working well `#305`
+- Some components unexpectedly can be added multiple times `#306`
 
 ### Security
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index dd83d67c1..fc3b9a3a2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,7 @@ The format is based on [Keep a Changelog].
 - Animating Behaviour.m_Enabled not working `#287`
 - Error Report Window may not refreshed after build error `#299`
 - Apply On Play may not working well `#305`
+- Some components unexpectedly can be added multiple times `#306`
 
 ### Security
 
diff --git a/Runtime/Activator.cs b/Runtime/Activator.cs
index dd59d5acb..658d105fe 100644
--- a/Runtime/Activator.cs
+++ b/Runtime/Activator.cs
@@ -13,6 +13,7 @@ namespace Anatawa12.AvatarOptimizer
     [AddComponentMenu("")]
     [ExecuteAlways]
     [DefaultExecutionOrder(-9989)]
+    [DisallowMultipleComponent]
     internal class Activator : MonoBehaviour
     {
         private void OnValidate()
@@ -34,6 +35,7 @@ private void RemoveSelf()
     [AddComponentMenu("")]
     [ExecuteAlways]
     [DefaultExecutionOrder(-9997)]
+    [DisallowMultipleComponent]
     internal class AvatarActivator : MonoBehaviour
     {
         private void Update()
diff --git a/Runtime/ClearEndpointPosition.cs b/Runtime/ClearEndpointPosition.cs
index e312f6649..05bce3ac3 100644
--- a/Runtime/ClearEndpointPosition.cs
+++ b/Runtime/ClearEndpointPosition.cs
@@ -5,6 +5,7 @@ namespace Anatawa12.AvatarOptimizer
 {
     [AddComponentMenu("Avatar Optimizer/AAO Clear Endpoint Position")]
     [RequireComponent(typeof(VRCPhysBoneBase))]
+    [DisallowMultipleComponent]
     internal class ClearEndpointPosition : AvatarTagComponent
     {
     }
diff --git a/Runtime/MakeChildren.cs b/Runtime/MakeChildren.cs
index 0348e740f..0f7f6bcf8 100644
--- a/Runtime/MakeChildren.cs
+++ b/Runtime/MakeChildren.cs
@@ -4,6 +4,7 @@
 namespace Anatawa12.AvatarOptimizer
 {
     [AddComponentMenu("Avatar Optimizer/AAO Make Children")]
+    [DisallowMultipleComponent]
     internal class MakeChildren : AvatarTagComponent
     {
         [CL4EELocalized("MakeChildren:prop:executeEarly", "MakeChildren:tooltip:executeEarly")]
diff --git a/Runtime/MergeToonLitMaterial.cs b/Runtime/MergeToonLitMaterial.cs
index ce45ea64d..fd7f89b55 100644
--- a/Runtime/MergeToonLitMaterial.cs
+++ b/Runtime/MergeToonLitMaterial.cs
@@ -4,6 +4,7 @@
 namespace Anatawa12.AvatarOptimizer
 {
     [AddComponentMenu("Avatar Optimizer/AAO Merge Toon Lit Material")]
+    [DisallowMultipleComponent]
     internal class MergeToonLitMaterial : EditSkinnedMeshComponent
     {
         public MergeInfo[] merges = Array.Empty<MergeInfo>();
diff --git a/Test~/AddComponentMenuTest.cs b/Test~/ComponentSettingsTest.cs
similarity index 72%
rename from Test~/AddComponentMenuTest.cs
rename to Test~/ComponentSettingsTest.cs
index a404db590..4ed03c23c 100644
--- a/Test~/AddComponentMenuTest.cs
+++ b/Test~/ComponentSettingsTest.cs
@@ -7,7 +7,7 @@
 
 namespace Anatawa12.AvatarOptimizer.Test
 {
-    public class AddComponentMenuTest
+    public class ComponentSettingsTest
     {
         [Test]
         [TestCaseSource(nameof(ComponentTypes))]
@@ -18,6 +18,14 @@ public void CheckAddComponentMenuIsInAvatarOptimizer(Type type)
             Assert.That(addComponentMenu.componentMenu, Does.StartWith("Avatar Optimizer/AAO ").Or.Empty);
         }
 
+        [Test]
+        [TestCaseSource(nameof(ComponentTypes))]
+        public void CheckDisallowMultipleComponentIsSpecified(Type type)
+        {
+            var addComponentMenu = type.GetCustomAttribute<DisallowMultipleComponent>();
+            Assert.That(addComponentMenu, Is.Not.Null);
+        }
+
         static IEnumerable<Type> ComponentTypes()
         {
             return 
diff --git a/Test~/AddComponentMenuTest.cs.meta b/Test~/ComponentSettingsTest.cs.meta
similarity index 100%
rename from Test~/AddComponentMenuTest.cs.meta
rename to Test~/ComponentSettingsTest.cs.meta