Unity Adding points to a user only once after collision - unity3d

Question

So I'm getting ready to create my first game, I just finished classes on the C# language so I apologize if I'm using stuff such as interfaces wrong and all that. However, for my question; I'm trying different things and seeing what works. I've created a coin, and a player. The coin works as it should, however sometimes when I collect it, it will give me twice the points it should. The coins value is 15, sometimes when I collect a coin it'll add 15 points, other times it will add 30. How do I prevent this from happening.
Here's my code:
Coin Controller Class:
public class CoinController : MonoBehaviour, IEconomy {
private int MoneyValue;
void Start () {
MoneyValue = 15;
}
void Update () {
}
void OnTriggerEnter(Collider col) {
if (col.CompareTag("Player")) {
Destroy(transform.gameObject);
Value();
}
}
public int Value() {
return EconomyController.Money += MoneyValue;
}
}
Economy Controller:
public class EconomyController : MonoBehaviour{
public static int Money;
void Start() {
Money = 0;
}
}
Economy Interface:
public interface IEconomy {
int Value();
}

Answer 1

I would like to point some things about your code:
A good practice when declaring variables is using lowerCamelCase:
thisIsLowerCamelCase
ThisIsNot
This is a variable name convention that is largely used in programming to differentiate Methods and Classes from variables.
Another thing I've noticed is that your "Money" variable is static and it is still being updated on your CoinController. I'd set this variable to be private int variable and use a setter to update it. With that in mind... Have you tried to use Debug.Log to check if the OnTriggerEnter is triggering twice before the object is destroyed?
Simply write:
Debug.Log ("This should only happen once!");
And play the game. If your console shows this message two times, this trigger is being called twice. Another thing that you might notice is that you are calling the Value () method after you called the Destroy (transform.gameObject).
I would do something like:
public class CoinController : MonoBehaviour{
private int moneyValue = 15;
private EconomyController economyController;
void Start (){
economyController = FindObjectOfType (typeof (EconomyController)) as EconomyController;
}
void OnTriggerEnter (Collider col) {
if (col.CompareTag("Player")) {
AddValue();
}
}
public int AddValue() {
EconomyController.money += moneyValue; //Option one.
EconomyController.AddMoney (moneyValue) ; //Option two.
DestroyGameObject ();
}
private void DestroyGameObject (){
Destroy(transform.gameObject);
}
}
Using the clean code principles, option 2 is using a public void function created inside the EconomyController class changing a private variable.

Answer 2

My intuition tells me that you are probably collecting two coins at the time. I'm not sure how you are setting out the coins but I've had a similar problem before.
Imagine a game of snake. Lets say you've programmed it so once you eat a square you create a new one to a random location. There is a probability that the new square would appear inside the snake so it would instantly be eaten. This could be why it happens only some of the time.

Answer 3

Try disabling the collider before you destroy it.
Destroying a gameobject isn't instant and it's (annoyingly) quite easy to set off triggers multiple times.
void OnTriggerEnter(Collider col) {
if (col.CompareTag("Player")) {
// Pseudo Code: GetComponent<TheColliderItIs>().Enabled = false;
Value();
Destroy(transform.gameObject);
}
}