LINQy way to check if any objects in a collection have the same property value

Question

I have a class Agent with a property Id

Given a collection of Agents I need to check if any of them have duplicate Ids.

I am currently doing this with a hash

Answer 1

foreach(var agent in Agents) {
    if(Agents.Count(a => a.ID == agent.ID) > 1)
        Console.WriteLine("Found: {0}", agent.ID);
}