Distinct

Returns unique values from an IEnumerable. Uniqueness is determined using the default equality comparer.

int[] array = { 1, 2, 3, 4, 2, 5, 3, 1, 2 };

var distinct = array.Distinct();
// distinct = { 1, 2, 3, 4, 5 }

To compare a custom data type, we need to implement the IEquatable<T> interface and provide GetHashCode and Equals methods for the type. Or the equality comparer may be overridden:

class SSNEqualityComparer : IEqualityComparer<Person> {
    public bool Equals(Person a, Person b) => return a.SSN == b.SSN;
    public int GetHashCode(Person p) => p.SSN;
}

List<Person> people;

distinct = people.Distinct(SSNEqualityComparer);

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