Add option to remove duplicate properties

[silentmatt]

This would allow you to override a class property without the overhead of having the same property twice. The problem with doing it by default is that it's not safe. For example:

selector {
    property: old-boring-style;
    property: super-new-css3-override;
}

The first one still gets applied on older browsers. Maybe this should be a syntax-level feature where you can explicitly override individual properties:

selector {
    property: this came from some class;

    !property: override; // ???
}