Deduplicate screen CSS errors.

Related issue: #3581.

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed `OptionList` event leakage from `CommandPalette` to `App`.
 - Fixed crash in `LoadingIndicator` https://github.com/Textualize/textual/pull/3498
 - Fixed crash when `Tabs` appeared as a descendant of `TabbedContent` in the DOM https://github.com/Textualize/textual/pull/3602
+- Duplicate CSS errors when parsing CSS from a screen https://github.com/Textualize/textual/issues/3581
 
 ### Added
 

src/textual/css/stylesheet.py

@@ -372,10 +372,19 @@ 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(