The problem with FindObjectOfType and Instantiate.Why is not one object created, but a multiplied copy?

It would be safer to have a boolean change when you press R and make Respawn a coroutine with a while loop.

 void Start()
     {
         StartCoroutine("Respawn");
     }
     void Update()
     {
   //One object is created directly
         if (Input.GetKeyDown(KeyCode.R))
         {
             respawnBool = true;
         }
     }
     IEnumerator Respawn()
     {
            while(true)
            {
                    yield return new WaitForSeconds(0.1f);
                    if(respawnBool)
                    {
                           float a = 0;
                           float b = 0;
                           Vector3 respawn = new Vector3(a, b, 0);
                           int i = Random.Range(0, groups.Length);
                           Instantiate(groups[i], respawn, Quaternion.identity);
                           respawnBool = false;
                    }
            }
     }