We call this a double definite. It's one of the differences between Swedish and Danish, we do this but they don't. In phrases with adjectives like de gamla städerna ('the old cities') or den gamla staden ('the old city'), the noun needs to be in the definite form, and there needs to be a definite article in front (except in some names etc).
Yes. There are some cases where expressions have become more or less names, where you can skip the article in determinate phrases with adjective + noun, but that's only in special combinations. One example of that is Vita huset 'The White House' which is a name and therefore is used without the article in Swedish.