Question About Unity Abstract Class Susbscribe an Action

I'm having a problem when subscribing an abstract class to an action in Unity. When I invoke the action, it sometimes executes more than once. However, when I subscribe a subclass to the action, it works correctly. What could be the reason for this?
This issue occurs when the user watches an ad and earns a reward.
I'm using Unity 2021.3.19.f1 LTS
Android platform
Here is my code:
SRTAdManager.cs

Code (CSharp):

1. public class SRTAdManager : MonoBehaviour
2. {
3.    
4. void Awake()
5.     {
6.         if (instance == null)
7.         {
8.             instance = this;
9.         }
10.         else {
11.              Destroy(this.gameObject);
12.         }
13.     }
14.     ...
15.     public event Action<string> onAdReward;
16.     public void UserEarnedReward(string rewardType)
17.     {
18.         onAdReward?.Invoke(rewardType);
19.     }
20.     ...
21. }

SRTAdmobAdManager.cs

Code (CSharp):

1. public class SRTAdmobAdManager : MonoBehaviour
2. {
3.     ...
4.     public void ShowAdmobRewarded(string userReward)
5.     {
6.         if (IsRewardedLoaded())
7.         {
8.             this.userReward = userReward;
9.             this.rewardedAd.Show();
10.         }
11.         else {
12.             LoadAdmobRewarded();
13.         }
14.     }
15.     public void HandleUserEarnedReward(object sender, Reward args)
16.     {
17.         SRTAdManager.instance.UserEarnedReward(userReward);
18.     }
19.     ...
20. }

RideUIManager.cs

Code (CSharp):

1. public abstract class RideUIManager : MonoBehaviour
2. {
3.     ...
4.     protected virtual void OnEnable()
5.     {
6.         SRTAdManager.instance.onAdReward += OnReward;
7.     }
8.     protected virtual void OnDestroy()
9.     {
10.         SRTAdManager.instance.onAdReward -= OnReward;
11.     }
12.     public abstract void OnReward(string rewardType);
13.     ...
14. }

AdvancedRideUIManager.cs

Code (CSharp):

1. public class AdvancedRideUIManager : RideUIManager
2. {
3.     protected override void OnEnable()
4.     {
5.         base.OnEnable();
6.     }
7.     protected override void OnDestroy()
8.     {
9.         base.OnDestroy();
10.     }
11.     public override void OnReward(string rewardType)
12.     {
13.         if (rewardType.Equals("MoneyBonusADV"))
14.         {
15.                 reward *= 2;
16.                 doubleItButton.SetActive(false);
17.                 rewardText.text = reward.ToString();
18.         }
19.         if (rewardType.Equals("BonusTimeADV"))
20.         {
21.                 loseModalWindowManager.ModalWindowOut();
22.                 advancedRide.advancedRide.remaningTime = rewardInfo.bonusTime;
23.                 advancedRide.hasTime = true;
24.                 advancedRide.enumared = false;
25.                 GameEventManager.GameEventPost(
26.                     "Bonus Time"
27.                 );
28.                 SetAudio(false);
29.                 Time.timeScale = 1;
30.         }
31.     }
32. }