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

this is how i would do it without the need to do group-by in one line:

 List duplicates = new HashSet(agents.Where(c => agents.Count(x => x.ID == c.ID) > 1)).ToList();