From 37aac01e93cd4d6a21142067edeba39c347efd6c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rodrigo=20Gir=C3=A3o=20Serr=C3=A3o?=
 <5621605+rodrigogiraoserrao@users.noreply.github.com>
Date: Mon, 30 Oct 2023 17:02:18 +0000
Subject: [PATCH] Deduplicate screen CSS errors.

Related issue: #3581.
---
 CHANGELOG.md                  |  1 +
 src/textual/css/stylesheet.py | 17 +++++++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf154b5ba8..5393e7a919 100644
--- a/CHANGELOG.md
+++ b/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
 
diff --git a/src/textual/css/stylesheet.py b/src/textual/css/stylesheet.py
index 3362520293..7852b2c1dd 100644
--- a/src/textual/css/stylesheet.py
+++ b/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(