diff --git a/docs/DuplicateKey.md b/docs/DuplicateKey.md
new file mode 100644
index 0000000..664ed3a
--- /dev/null
+++ b/docs/DuplicateKey.md
@@ -0,0 +1,23 @@
+# DuplicateKey
+
+If the keys of the array duplicate, it will be overwritten with the specified value later.
+
+NOTE: Array keys are cast in some cases. However, it is not possible to detect duplicate keys in such cases.
+
+## Before
+
+```php
+<?php
+$array = ["a" => 1, "a" => 2]; // DuplicateKey: Duplicate keys found in array.
+```
+
+## After
+
+```php
+<?php
+$array = ["a" => 1, "b" => 2]; // OK!
+```
+
+## Reference
+
+https://secure.php.net/manual/en/language.types.array.php
diff --git a/docs/README.md b/docs/README.md
index f7777a6..3cd846c 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -35,3 +35,4 @@
 
 - [SquareBracketSyntax](SquareBracketSyntax.md)
 - [RedundantTernaryOperator](RedundantTernaryOperator.md)
+- [DuplicateKey](DuplicateKey.md)
diff --git a/src/Tool/DuplicateKey.php b/src/Tool/DuplicateKey.php
new file mode 100644
index 0000000..0f768d4
--- /dev/null
+++ b/src/Tool/DuplicateKey.php
@@ -0,0 +1,85 @@
+<?php declare(strict_types=1);
+
+namespace Pahout\Tool;
+
+use \ast\Node;
+use Pahout\Logger;
+use Pahout\Hint;
+
+/**
+* Detect duplicate key in array.
+*
+* If the keys of the array duplicate, it will be overwritten with the specified value later.
+*
+* ## Before
+*
+* ```
+* <?php
+* $array = ["a" => 1, "a" => 2];
+* ```
+*
+* ## After
+*
+* ```
+* <?php
+* $array = ["a" => 1, "b" => 2];
+* ```
+*
+*/
+class DuplicateKey implements Base
+{
+    /** Analyze array declarations node (AST_ARRAY) */
+    public const ENTRY_POINT = \ast\AST_ARRAY;
+    /** PHP version to enable this tool */
+    public const PHP_VERSION = '0.0.0';
+    public const HINT_TYPE = "DuplicateKey";
+    private const HINT_MESSAGE = 'Duplicate keys found in array.';
+    private const HINT_LINK = Hint::DOCUMENT_LINK."/DuplicateKey.md";
+
+    /**
+    * Detect duplicate numbers, strings, node key
+    *
+    * @param string $file File name to be analyzed.
+    * @param Node   $node AST node to be analyzed.
+    * @return Hint[] List of hints obtained from results.
+    */
+    public function run(string $file, Node $node): array
+    {
+        $hints = [];
+        $keys = [];
+        $elems = [];
+
+        foreach ($node->children as $elem) {
+            if ($elem instanceof Node && !is_null($elem->children['key'])) {
+                $elems[] = $elem;
+                $key = $elem->children['key'];
+                $keys[] = $key instanceof Node ? $key->children : $key;
+            }
+        }
+
+        while (count($keys) > 0) {
+            $elem = array_pop($elems);
+            $key = array_pop($keys);
+
+            if ($key instanceof Node) {
+                // If the key is a node, use equal comparison operator
+                $strict = false;
+            } else {
+                // If the key is not a node, use identical comparison operator
+                $strict = true;
+            }
+
+            if (in_array($key, $keys, $strict)) {
+                $hints[] = new Hint(
+                    self::HINT_TYPE,
+                    self::HINT_MESSAGE,
+                    $file,
+                    $elem->lineno,
+                    self::HINT_LINK
+                );
+            }
+        }
+
+        return $hints;
+    }
+}
diff --git a/src/ToolBox.php b/src/ToolBox.php
index 397283c..cf45ec6 100644
--- a/src/ToolBox.php
+++ b/src/ToolBox.php
@@ -24,6 +24,7 @@ class ToolBox
         'EscapeShellArg',
         'SquareBracketSyntax',
         'RedundantTernaryOperator',
+        'DuplicateKey',
     ];
 
     /**
diff --git a/tests/Tool/DuplicateKeyTest.php b/tests/Tool/DuplicateKeyTest.php
new file mode 100644
index 0000000..620e246
--- /dev/null
+++ b/tests/Tool/DuplicateKeyTest.php
@@ -0,0 +1,92 @@
+<?php
+
+namespace Pahout\Test\Tool;
+
+use PHPUnit\Framework\TestCase;
+use Pahout\Test\helper\PahoutHelper;
+use Pahout\Tool\DuplicateKey;
+use Pahout\Hint;
+use Pahout\Logger;
+use Pahout\Config;
+use Symfony\Component\Console\Output\ConsoleOutput;
+
+class DuplicateKeyTest extends TestCase
+{
+    public function setUp()
+    {
+        Logger::getInstance(new ConsoleOutput());
+    }
+
+    public function test_duplicate_key()
+    {
+        $code = <<<'CODE'
+<?php
+$array = [
+    "a" => 1,
+    "a" => 2,
+];
+CODE;
+        $root = \ast\parse_code($code, Config::AST_VERSION);
+
+        $tester = PahoutHelper::create(new DuplicateKey());
+        $tester->test($root);
+
+        $this->assertEquals(
+            [
+                new Hint(
+                    'DuplicateKey',
+                    'Duplicate keys found in array.',
+                    './test.php',
+                    4,
+                    Hint::DOCUMENT_LINK.'/DuplicateKey.md'
+                )
+            ],
+            $tester->hints
+        );
+    }
+
+    public function test_duplicate_node_key()
+    {
+        $code = <<<'CODE'
+<?php
+$array = [
+    $test => 1,
+    $test => 2
+];
+CODE;
+        $root = \ast\parse_code($code, Config::AST_VERSION);
+
+        $tester = PahoutHelper::create(new DuplicateKey());
+        $tester->test($root);
+
+        $this->assertEquals(
+            [
+                new Hint(
+                    'DuplicateKey',
+                    'Duplicate keys found in array.',
+                    './test.php',
+                    4,
+                    Hint::DOCUMENT_LINK.'/DuplicateKey.md'
+                )
+            ],
+            $tester->hints
+        );
+    }
+
+    public function test_unduplicate_key()
+    {
+        $code = <<<'CODE'
+<?php
+$array = [
+    "a" => 1,
+    "b" => 2
+];
+CODE;
+        $root = \ast\parse_code($code, Config::AST_VERSION);
+
+        $tester = PahoutHelper::create(new DuplicateKey());
+        $tester->test($root);
+
+        $this->assertEmpty($tester->hints);
+    }
+}