How do you check if a collection contains duplicates?

var query = lst.GroupBy(x => x)
              .Where(g => g.Count() > 1)
              .Select(y => y.Key)
              .ToList();

better don't allow to add duplicates, there is 'contains' utilize it or use collection which doesn't allow duplicates

bool isAnyDuplicate=YourList.GroupBy(x =>x.Name).Any(c => c.Count() > 1);
//Name is property in your class

for complex type use group by as Distinct won't work without IComparer implementation

You can try the following code to check wheather a collection have duplicate value or not.

var yourList = new List();

yourList.Add(1);
yourList.Add(2);
yourList.Add(3);
yourList.Add(1);

if (yourList.Count != yourList.Distinct().Count())
{
	// duplicates values exist in list
}