Duplicate CSS errors from screens

[rodrigogiraoserrao]

CSS errors in screens are still duplicate (see #3566, and https://github.com/Textualize/textual/pull/3566/files#r1367884524 in particular).

CSS parsing errors show up as duplicate when parsing the CSS class variable or css from an external file, but not from DEFAULT_CSS.

App to reproduce:

from textual.app import App
from textual.screen import Screen


class MyScreen(Screen[None]):
    # CSS_PATH = "myscreen.tcss"
    CSS = "Screen { screen-baz: spam; }"


class MyApp(App[None]):
    def on_mount(self):
        self.push_screen(MyScreen())


if __name__ == "__main__":
    MyApp().run()

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