Deduplicate screen CSS errors.

CHANGELOG.md

@@ -8,6 +8,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+
+- Duplicate CSS errors when parsing CSS from a screen https://github.com/Textualize/textual/issues/3581
+
 ### Changed
 
 - CSS error reporting will no longer provide links to the files in question https://github.com/Textualize/textual/pull/3582

src/textual/css/stylesheet.py

@@ -389,10 +389,19 @@ def reparse(self) -> None:
                 tie_breaker=tie_breaker,
                 scope=scope,
             )
-        stylesheet.parse()
-        self._rules = stylesheet.rules
-        self._rules_map = None
-        self.source = stylesheet.source
+        try:
+            stylesheet.parse()
+        except Exception:
+            # If we don't update self's invalid CSS, we might end up reparsing this CSS
+            # before Textual quits application mode.
+            # See https://github.com/Textualize/textual/issues/3581.
+            self._invalid_css.update(stylesheet._invalid_css)
+            raise
+        else:
+            self._rules = stylesheet.rules
+            self._rules_map = None
+            self.source = stylesheet.source
+            self._require_parse = False
 
     @classmethod
     def _check_rule(