Unique

var lisa = new Person { Name = "Lisa" };
var bart = new Person { Name = "Bart" };
var maggie = new Person { Name = "Maggie" };
var simpsonsKids = new List<Person> { bart, lisa, maggie, maggie };
simpsonsKids.ShouldBeUnique();

snippet source | anchor

Exception

simpsonsKids
    should be unique but
[Maggie]
    was duplicated