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I am looking for a really fast way to check for duplicates in a list of objects. I was thinking of simply looping through the list and doing a manual comparison that way, but I thought that linq might provide a more elegant solution... Suppose I have an object... public class dupeCheckee { public string checkThis { get; set; } public string checkThat { get; set; } dupeCheckee(string val, string val2) { checkThis = val; checkThat = val2; } } And I have a list of those objects List<dupeCheckee> dupList = new List<dupeCheckee>(); dupList.Add(new dupeCheckee("test1", "value1")); dupList.Add(new dupeCheckee("test2", "value1")); dupList.Add(new dupeCheckee("test3", "value1")); dupList.Add(new dupeCheckee("test1", "value1"));//dupe dupList.Add(new dupeCheckee("test2", "value1"));//dupe... dupList.Add(new dupeCheckee("test4", "value1")); dupList.Add(new dupeCheckee("test5", "value1")); dupList.Add(new dupeCheckee("test1", "value2"));//not dupe I need to find the dupes in that list. When I find it, I need to do some additional logic not necessarily removing them. When I use linq some how my GroupBy is throwing an exception... 'System.Collections.Generic.List<dupeCheckee>' does not contain a definition for 'GroupBy' and no extension method 'GroupBy' accepting a first argument of type 'System.Collections.Generic.List<dupeCheckee>' could be found (are you missing a using directive or an assembly reference?) Which is telling me that I am missing a library. I am having a hard time figuring out which one though. Once I figure that out though, How would I essentially check for those two conditions... IE checkThis and checkThat both occur more than once? UPDATE: What I came up with This is the linq query that I came up with after doing quick

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