Temporary Buff scripts and timers

Hi all, I am using the following script to add a set amount of health to the player for 30 seconds.

When I press "2" to test it the script works fine, I see the buff apply, then disappear, however if I run it again the buff applies but never disappears again (its like the timer only functions once)

Also, I cant figure out how to stop this from stacking if I keep triggering it.

Any help on this would be greatly appreciated :)

 using UnityEngine;
 using System.Collections;
 
 public class ScriptTempHealthBuff : MonoBehaviour {
 
     ScriptPlayerOneManager script;
   
     public int healthBuffAmount = 30;
     public int healthBuffDuration = 5;
 
     // Use this for initialization
     void Start () {
     script = GameObject.Find("GameObjectPlayerOne").GetComponent<ScriptPlayerOneManager>();
     StartCoroutine(playerHealthBuffTimer());
 
     }
     
     // Update is called once per frame
     void Update () {
 
         if (Input.GetKeyDown("2"))
         {
         script.playerTempAttributes.playerOneTempHealth += healthBuffAmount;
         playerHealthBuffTimer();
         }
      
     }
 
     // Now run a timer
     IEnumerator playerHealthBuffTimer()
     {
         yield return new WaitForSeconds(healthBuffDuration);
         playerHealthBuffUndo();
     }
 
     //Now return the value back to original
     void playerHealthBuffUndo()
     {
         script.playerTempAttributes.playerOneTempHealth -= healthBuffAmount;
     }
 }