diff --git a/src/TomlKeystore.php b/src/TomlKeystore.php
index 1b86cbe..44c3233 100644
--- a/src/TomlKeystore.php
+++ b/src/TomlKeystore.php
@@ -242,7 +242,7 @@ protected function addArrayTableNode(ArrayTableNode $arrayTableNode): void
         }
 
         if ($index === 0 && ! $this->tables->filter(
-            static fn ($table) => str_starts_with((string) $table, $header))->isEmpty()
+            static fn ($table) => str_starts_with((string) $table, $header  . "."))->isEmpty()
         ) {
             throw new TomlError('key duplication');
         }
diff --git a/tests/Unit/TomlDecodeTest.php b/tests/Unit/TomlDecodeTest.php
index e0060be..2eed11b 100644
--- a/tests/Unit/TomlDecodeTest.php
+++ b/tests/Unit/TomlDecodeTest.php
@@ -63,6 +63,11 @@ function () {
         { "name": "plantain" }
       ]
     }
+  ],
+  "fruit": [
+    {
+      "name": "strawberry"
+    }
   ]
 }
 JSON_STRING;
@@ -111,6 +116,9 @@ function () {
 
 [[fruits.varieties]]
 name = "plantain"
+
+[[fruit]] # prefix of existing array
+name = "strawberry"
 TOML_STRING;
 
         expect(toml_decode($toml))->toEqual(json_decode($json, false))
diff --git a/tests/Unit/TomlErrorTest.php b/tests/Unit/TomlErrorTest.php
index f8628c5..5dbdd05 100644
--- a/tests/Unit/TomlErrorTest.php
+++ b/tests/Unit/TomlErrorTest.php
@@ -35,3 +35,31 @@ function () {
 
         expect(static fn () => toml_decode($toml, true))->toThrow(TomlError::class, $message);
     });
+
+it('throws on invalid array order',
+    /**
+     * @throws TomlError
+     */
+    function () {
+
+        $message = <<<'MESSAGE'
+Invalid TOML document: key duplication
+
+5:  # This requires a to be an array table
+6:  [[a]]
+         ^
+7:  y = 2
+MESSAGE;
+
+        $toml = <<<'TOML_STRING'
+# This creates a as a normal (non-array) table
+[[a.b]]
+x = 1
+
+# This requires a to be an array table
+[[a]]
+y = 2
+TOML_STRING;
+
+        expect(static fn () => toml_decode($toml, true))->toThrow(TomlError::class, $message);
+    });