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

Similar to Y Low's approach,

Edited:

 var duplicates = agents.GroupBy(a => a.ID).Where(a=>a.Count() > 1);

 foreach (var agent in duplicates)
 {
         Console.WriteLine(agent.Key.ToString());
 }