diff --git a/README.md b/README.md
index 5c8b83cb..23efb298 100644
--- a/README.md
+++ b/README.md
@@ -417,3 +417,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 2335. Minimum Amount of Time to Fill Cups                          | [Link](https://leetcode.com/problems/minimum-amount-of-time-to-fill-cups/)                          | [Link](./lib/easy/2335_minimum_amount_of_time_to_fill_cups.rb)                          |
 | 2341. Maximum Number of Pairs in Array                             | [Link](https://leetcode.com/problems/maximum-number-of-pairs-in-array/)                             | [Link](./lib/easy/2341_maximum_number_of_pairs_in_array.rb)                             |
 | 2347. Best Poker Hand                                              | [Link](https://leetcode.com/problems/best-poker-hand/)                                              | [Link](./lib/easy/2347_best_poker_hand.rb)                                              |
+| 2351. First Letter to Appear Twice                                 | [Link](https://leetcode.com/problems/first-letter-to-appear-twice/)                                 | [Link](./lib/easy/2351_first_letter_to_appear_twice.rb)                                 |
diff --git a/leetcode-ruby.gemspec b/leetcode-ruby.gemspec
index 93169db0..ff8d7c70 100644
--- a/leetcode-ruby.gemspec
+++ b/leetcode-ruby.gemspec
@@ -5,7 +5,7 @@ require 'English'
 ::Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
   s.name = 'leetcode-ruby'
-  s.version = '5.5.6'
+  s.version = '5.5.7'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[bin/leetcode-ruby README.md LICENSE]
   s.executable = 'leetcode-ruby'
diff --git a/lib/easy/2351_first_letter_to_appear_twice.rb b/lib/easy/2351_first_letter_to_appear_twice.rb
new file mode 100644
index 00000000..48f53a23
--- /dev/null
+++ b/lib/easy/2351_first_letter_to_appear_twice.rb
@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/first-letter-to-appear-twice/
+# @param {String} s
+# @return {Character}
+def repeated_character(s)
+  letters = ::Array.new(128, 0)
+  s.each_char do |c|
+    return c if letters[c.ord] == 1
+
+    letters[c.ord] += 1
+  end
+
+  '0'
+end
diff --git a/test/easy/test_2351_first_letter_to_appear_twice.rb b/test/easy/test_2351_first_letter_to_appear_twice.rb
new file mode 100644
index 00000000..08ff32ce
--- /dev/null
+++ b/test/easy/test_2351_first_letter_to_appear_twice.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/easy/2351_first_letter_to_appear_twice'
+require 'minitest/autorun'
+
+class FirstLetterToAppearTwiceTest < ::Minitest::Test
+  def test_default
+    assert_equal('c', repeated_character('abccbaacz'))
+    assert_equal('d', repeated_character('abcdd'))
+  end
+
+  def test_additional
+    assert_equal('0', repeated_character('abc'))
+  end
+end