MySQL Case sensitive insert

I have created a database with case sensitive character set

Create database Grupo88 character set utf8 collate utf8_bin;

CREATE TABLE Grupo88.Usuarios(
    nombreUsuario varchar(30) primary key,
    clave varchar(120) not null);

INSERT INTO usuarios(nombreUsuario,clave)
       VALUES('guy','pass');
INSERT INTO usuario(nombreUsuario, clave)
       VALUES('Guy', 'password');

The first insert goes well, but the second one says that the value "Guy" already exists. Setting my database to be case sensitive is not enough? How can I do to allow case sensitive inserts?

Thanks