Task identifiers are not unique. On my machine, this program will run for 3 minutes and then produce the output.

Output

Id collision!
  task.Id == other.Id: True
  task == other: False

Program.cs

using System;
using System.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        var task = new TaskCompletionSource<object>(null).Task;
        var taskId = task.Id;

        Task other;
        do
        {
            other = new TaskCompletionSource<object>(null).Task;
            if (other.Id == 0)
                Console.WriteLine("Saw Id of 0!"); // (never executed)
        } while (other.Id != task.Id);

        Console.WriteLine("Id collision!");
        Console.WriteLine("  task.Id == other.Id: " + (task.Id == other.Id));
        Console.WriteLine("  task == other: " + (task == other));
        Console.ReadKey();
    }
}