diff --git a/src/Expression/ForClasses/ResideInOneOfTheseNamespaces.php b/src/Expression/ForClasses/ResideInOneOfTheseNamespaces.php
index 5048486d..4b5978ba 100644
--- a/src/Expression/ForClasses/ResideInOneOfTheseNamespaces.php
+++ b/src/Expression/ForClasses/ResideInOneOfTheseNamespaces.php
@@ -18,7 +18,7 @@ class ResideInOneOfTheseNamespaces implements Expression
 
     public function __construct(string ...$namespaces)
     {
-        $this->namespaces = $namespaces;
+        $this->namespaces = array_values(array_unique($namespaces));
     }
 
     public function describe(ClassDescription $theClass, string $because): Description
diff --git a/tests/Unit/Expressions/ForClasses/ResideInOneOfTheseNamespacesTest.php b/tests/Unit/Expressions/ForClasses/ResideInOneOfTheseNamespacesTest.php
index 95b526ff..4f8dce9e 100644
--- a/tests/Unit/Expressions/ForClasses/ResideInOneOfTheseNamespacesTest.php
+++ b/tests/Unit/Expressions/ForClasses/ResideInOneOfTheseNamespacesTest.php
@@ -84,4 +84,14 @@ public function test_it_should_check_multiple_namespaces_in_or(): void
         $haveNameMatching->evaluate($classDesc, $violations, $because);
         self::assertNotEquals(0, $violations->count());
     }
+
+    public function test_duplicate_namespaces_are_removed(): void
+    {
+        $expression = new ResideInOneOfTheseNamespaces('A', 'B', 'A', 'C', 'D', 'D');
+
+        self::assertSame(
+            'should reside in one of these namespaces: A, B, C, D because rave',
+            $expression->describe(ClassDescription::getBuilder('Marko')->build(), 'rave')->toString()
+        );
+    }
 }