diff --git a/cssutils/util.py b/cssutils/util.py
index 66c43e45..c21ec90c 100644
--- a/cssutils/util.py
+++ b/cssutils/util.py
@@ -3,10 +3,13 @@
 __all__ = []
 
 import codecs
+import operator
 import re
 import xml.dom
 from itertools import chain
 
+from more_itertools import unique_everseen
+
 import cssutils
 
 from . import codec, errorhandler, tokenize2
@@ -827,14 +830,15 @@ def namespaces(self):
         A property holding only effective @namespace rules in
         self.parentStyleSheets.
         """
-        namespaces = {}
-        for rule in filter(
+        ns_rules = filter(
             lambda r: r.type == r.NAMESPACE_RULE,
             reversed(self.parentStyleSheet.cssRules),
-        ):
-            if rule.namespaceURI not in list(namespaces.values()):
-                namespaces[rule.prefix] = rule.namespaceURI
-        return namespaces
+        )
+        unique_rules = unique_everseen(
+            ns_rules,
+            key=operator.attrgetter('namespaceURI'),
+        )
+        return {rule.prefix: rule.namespaceURI for rule in unique_rules}
 
     def get(self, prefix, default):
         return self.namespaces.get(prefix, default)
diff --git a/pyproject.toml b/pyproject.toml
index 4a721a3b..ceb5d4f6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,6 +25,7 @@ classifiers = [
 keywords = ["CSS", "Cascading Style Sheets", "CSSParser", "DOM Level 2 Stylesheets", "DOM Level 2 CSS"]
 requires-python = ">=3.8"
 dependencies = [
+	"more_itertools",
 ]
 dynamic = ["version"]