Hy, i would try to do something like this in your enemy script:
bool isAlive = true;
int health = 100;
public void GetDamage(int dmg, int playerID = 1)
{
health -= dmg;
if (isAlive && health <= 0)
{
ScoreManager.SetScore(playerID, 100);
isAlive = false;
Destroy(gameObject);
}
}
This way, its not possible to set the score twice, even if the hits happen at the same frame.