Except.cs

namespace ExceptIsInteresting
{
	class Program
	{
		static void Main(string[] args)
		{
			var xs = new[] { "a", "a", "b" };
			var ys = new[] { "b" };

			var result = xs.Except(ys);

			Console.WriteLine(string.Join( ", ", result.ToArray() ));

			// you'd expect the output to be `a, a`
			// but you actually get `a`.
		}
	}
}