diff --git a/CHANGELOG.md b/CHANGELOG.md
index 611f702a..e1608818 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,8 @@ All notable changes to this project will be documented in this file, in reverse
 
 - [#41](https://github.com/webimpress/coding-standard/pull/41) adds `ControlStructures\DefaultAsLast` sniff which requires `default` case to be last case in a switch control structure
 
+- [#39](https://github.com/webimpress/coding-standard/pull/39) adds `Arrays\DuplicateKey` sniff which detects duplicated keys in arrays
+
 ### Changed
 
 - Nothing.
diff --git a/src/WebimpressCodingStandard/Sniffs/Arrays/DuplicateKeySniff.php b/src/WebimpressCodingStandard/Sniffs/Arrays/DuplicateKeySniff.php
new file mode 100644
index 00000000..cee1f101
--- /dev/null
+++ b/src/WebimpressCodingStandard/Sniffs/Arrays/DuplicateKeySniff.php
@@ -0,0 +1,129 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WebimpressCodingStandard\Sniffs\Arrays;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\AbstractArraySniff;
+use PHP_CodeSniffer\Util\Tokens;
+
+use function in_array;
+use function is_array;
+use function preg_match;
+use function preg_replace;
+use function strtr;
+use function substr;
+use function token_get_all;
+
+use const T_CONSTANT_ENCAPSED_STRING;
+use const T_DOUBLE_QUOTED_STRING;
+use const T_VARIABLE;
+
+class DuplicateKeySniff extends AbstractArraySniff
+{
+    private const ALLOWED_CHARS = '/(?<!\\\\)(?:\\\\{2})*\\\(?:[0-7nrftve]|x[A-Fa-f0-9])/';
+
+    /**
+     * @param File $phpcsFile
+     * @param int $stackPtr
+     * @param int $arrayStart
+     * @param int $arrayEnd
+     * @param array $indices
+     */
+    protected function processSingleLineArray($phpcsFile, $stackPtr, $arrayStart, $arrayEnd, $indices) : void
+    {
+        $this->processArray($phpcsFile, $indices);
+    }
+
+    /**
+     * @param File $phpcsFile
+     * @param int $stackPtr
+     * @param int $arrayStart
+     * @param int $arrayEnd
+     * @param array $indices
+     */
+    protected function processMultiLineArray($phpcsFile, $stackPtr, $arrayStart, $arrayEnd, $indices) : void
+    {
+        $this->processArray($phpcsFile, $indices);
+    }
+
+    private function processArray(File $phpcsFile, array $indices) : void
+    {
+        $tokens = $phpcsFile->getTokens();
+
+        $keys = [];
+        foreach ($indices as $element) {
+            if (! isset($element['index_start']) || ! isset($element['index_end'])) {
+                continue;
+            }
+
+            $key = '';
+            for ($i = $element['index_start']; $i <= $element['index_end']; ++$i) {
+                $token = $tokens[$i];
+
+                if ($token['code'] === T_DOUBLE_QUOTED_STRING
+                    || $token['code'] === T_CONSTANT_ENCAPSED_STRING
+                ) {
+                    $content = '';
+                    do {
+                        $content .= $tokens[$i]['content'];
+
+                        if (! isset($tokens[$i + 1])
+                            || $tokens[$i + 1]['code'] !== $token['code']
+                        ) {
+                            break;
+                        }
+                    } while (++$i);
+
+                    $key .= $this->doubleQuoted($content);
+                } elseif (! in_array($token['code'], Tokens::$emptyTokens, true)) {
+                    $key .= $token['content'];
+                }
+            }
+
+            if (isset($keys[$key])) {
+                $phpcsFile->addError(
+                    'Duplicated array key; first usage in line %d',
+                    $element['index_start'],
+                    'DuplicateKey',
+                    [$keys[$key]]
+                );
+            } else {
+                $keys[$key] = $tokens[$element['index_start']]['line'];
+            }
+        }
+    }
+
+    private function doubleQuoted(string $string) : string
+    {
+        if ($string[0] !== '"' || substr($string, -1) !== '"') {
+            return $this->preg($string);
+        }
+
+        $tokens = token_get_all('<?php ' . $string);
+        foreach ($tokens as $token) {
+            if (is_array($token) && $token[0] === T_VARIABLE) {
+                return $this->preg($string, true);
+            }
+        }
+
+        if (preg_match(self::ALLOWED_CHARS, $string)) {
+            return $this->preg($string, true);
+        }
+
+        $string = strtr(substr($string, 1, -1), [
+            '\\"' => '"',
+            "'" => "\\'",
+        ]);
+
+        return "'" . $this->preg($string) . "'";
+    }
+
+    private function preg(string $string, bool $doubleQuotes = false) : string
+    {
+        $chars = $doubleQuotes ? '[\\\\"0-7nrftve]|x[A-Fa-f0-9]' : '[\\\\\']';
+
+        return preg_replace('/(?<!\\\\)(?:\\\\{2})*\\\(?!' . $chars . ')/', '$0\\\\', $string);
+    }
+}
diff --git a/test/Sniffs/Arrays/DuplicateKeyUnitTest.inc b/test/Sniffs/Arrays/DuplicateKeyUnitTest.inc
new file mode 100644
index 00000000..e07ee7c0
--- /dev/null
+++ b/test/Sniffs/Arrays/DuplicateKeyUnitTest.inc
@@ -0,0 +1,139 @@
+<?php
+
+$inline = ['a' => 1, 'b' => 2, 'a' => 3];
+
+$multiline = [
+    PHP_INT_MAX => 0,
+    'ab' => 1,
+    'b' => 2,
+    "b" => 3,
+    'a'.'b' => 4,
+    'a'. // comment
+    'b' => 5,
+    'noindex',
+    'a\\b' => 6,
+    'a\b' => 7,
+    'a\\\\b' => 8,
+    'a\\\b' => 9,
+    "a\b" => 10,
+    "a\\b" => 11,
+    'a\'b' => 12,
+    "a'b" => 13,
+    'b$a' => 14,
+    "b$a" => 15,
+    'a
+    b' => 16,
+    "a
+    b" => 17,
+    'PHP_INT_MAX' => 18,
+];
+
+$a = [
+    '\a' => 1,
+    '\\a' => 2,
+    "\a" => 3,
+    "\\a" => 4,
+];
+
+$b = [
+    '\b' => 1,
+    '\\b' => 2,
+    "\b" => 3,
+    "\\b" => 4,
+];
+
+$e = [
+    '\e' => 1,
+    '\\e' => 2,
+    "\e" => 3,
+    "\\e" => 4,
+];
+
+$f = [
+    '\f' => 1,
+    '\\f' => 2,
+    "\f" => 3,
+    "\\f" => 4,
+];
+
+$n = [
+    '\n' => 1,
+    '\\n' => 2,
+    "\n" => 3,
+    "\\n" => 4,
+];
+
+$r = [
+    '\r' => 1,
+    '\\r' => 2,
+    "\r" => 3,
+    "\\r" => 4,
+];
+
+$t = [
+    '\t' => 1,
+    '\\t' => 2,
+    "\t" => 3,
+    "\\t" => 4,
+];
+
+$v = [
+    '\v' => 1,
+    '\\v' => 2,
+    "\v" => 3,
+    "\\v" => 4,
+];
+
+$x = [
+    '\x' => 1,
+    '\\x' => 2,
+    "\x" => 3,
+    "\\x" => 4,
+
+    '\xa' => 1,
+    '\\xa' => 2,
+    "\xa" => 3,
+    "\\xa" => 4,
+
+    '\xF' => 1,
+    '\\xF' => 2,
+    "\xF" => 3,
+    "\\xF" => 4,
+
+    '\x0' => 1,
+    '\\x0' => 2,
+    "\x0" => 3,
+    "\\x0" => 4,
+
+    '\x9' => 1,
+    '\\x9' => 2,
+    "\x9" => 3,
+    "\\x9" => 4,
+
+    '\xZ' => 1,
+    '\\xZ' => 2,
+    "\xZ" => 3,
+    "\\xZ" => 4,
+];
+
+$quote = [
+    '"' => 1,
+    "\"" => 2,
+
+    "'" => 1,
+    '\'' => 2,
+];
+
+$slashes1 = [
+    'a\\b' => 1,
+    'a\b' => 2,
+    'a\\\b' => 3,
+    'a\\\\b' => 4,
+];
+
+$slashes2 = [
+    "a\\b" => 1,
+    "a\b" => 2,
+    "a\\\b" => 3,
+    "a\\\\b" => 4,
+];
diff --git a/test/Sniffs/Arrays/DuplicateKeyUnitTest.php b/test/Sniffs/Arrays/DuplicateKeyUnitTest.php
new file mode 100644
index 00000000..a55633ac
--- /dev/null
+++ b/test/Sniffs/Arrays/DuplicateKeyUnitTest.php
@@ -0,0 +1,68 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WebimpressCodingStandardTest\Sniffs\Arrays;
+
+use WebimpressCodingStandardTest\Sniffs\AbstractTestCase;
+
+class DuplicateKeyUnitTest extends AbstractTestCase
+{
+    protected function getErrorList(string $testFile = '') : array
+    {
+        return [
+            3 => 1,
+            9 => 1,
+            11 => 1,
+            15 => 1,
+            17 => 1,
+            18 => 1,
+            19 => 1,
+            21 => 1,
+            26 => 1,
+            33 => 1,
+            34 => 1,
+            35 => 1,
+            40 => 1,
+            41 => 1,
+            42 => 1,
+            47 => 1,
+            49 => 1,
+            54 => 1,
+            56 => 1,
+            61 => 1,
+            63 => 1,
+            68 => 1,
+            70 => 1,
+            75 => 1,
+            77 => 1,
+            82 => 1,
+            84 => 1,
+            89 => 1,
+            90 => 1,
+            91 => 1,
+            94 => 1,
+            96 => 1,
+            99 => 1,
+            101 => 1,
+            104 => 1,
+            106 => 1,
+            109 => 1,
+            111 => 1,
+            114 => 1,
+            115 => 1,
+            116 => 1,
+            121 => 1,
+            124 => 1,
+            129 => 1,
+            131 => 1,
+            136 => 1,
+            138 => 1,
+        ];
+    }
+
+    protected function getWarningList(string $testFile = '') : array
+    {
+        return [];
+    }
+}