Make sure your object's class overrides Equals and GetHashCode and then you can pass the List<T> to HashSet<T> constructor.

var hashSet = new HashSet<YourType>(yourList);

You may see: What is the best algorithm for an overridden System.Object.GetHashCode?

An alternative way would be

var yourlist = new List<SomeClass>();

// [...]

var uniqueObjs = yourlist.Distinct();  //Gives you a List with unique Objects of the List.

Note that this is only possible, if SomeClass overrides GetHashCode and Equals in some way. This is also true for

var uniqueObjs = new HashSet<SomeType>(yourOriginalList);

Otherwise you could implement you own IEqualityComnparer-class and pass it to distinct.

Note that with the Distinct() approach, you can also look for distinct property values of the object in the list:

var uniqueNames = yourlist.Select(obj => obj.Name).Distinct(); 

and some more...

If your type implements IEquatable<T>, Equals() and GetHashCode() correctly, then you don't need to do the de-duplication yourself. You can use Linq's Distinct() to do so like this:

myList = myList.Distinct().ToList();