duck

public class Inventory
{
    private List<Item> contents = new List<Item>();

    public bool Add(Item item)
    {
        // returns true for success, false if could not add

        if (contents.Contains(item))
        {
            Debug.Log("Cannot add "+item.name+" to inventory. An equal item is already present!");
            return false;
        }
        else
        {
            Console.WriteLine("Adding "+item.name+" to inventory");
            contents.Add(item);
            return true;
        }
    }
}

public class Item : IEquatable<Item>
{

    private static int idIndex = 0;
    public string name;

    public Item(string name)
    {
        this.name = name;
        ID = idIndex++;
    }

    public bool Equals(Item other)
    {
        return (this.ID == other.ID);
    }
}