Why uniq do not work with array of strings?

[Suliman via Digitalmars-d-learn]

The question appear here
http://stackoverflow.com/questions/28546572/how-to-find-duplicates-in-array-of-strings-in-d
I can't understand, why uniq work for array of int but do not 
work with array of strings.

        int[] arr = [ 1, 2, 2, 2, 2, 3, 4, 4, 4, 5 ];
        writeln(uniq(arr));

        string [] str = ["qwe","asd","zxc", "qwe"];
        auto uniqarr = uniq(str);

        foreach(x;uniqarr)
        {
                writeln(x);
        }

Running .\test.exe
[1, 2, 3, 4, 5]
qwe
asd
zxc
qwe

^ "qwe" prints two times.