Use set

.gitignore

@@ -0,0 +1 @@
+test/*

level0

@@ -1,12 +1,13 @@
 #!/usr/bin/env ruby
+require 'set'
 
 # Our test cases will always use the same dictionary file (with SHA1
 # 6b898d7c48630be05b72b3ae07c5be6617f90d8e). Running `test/harness`
 # will automatically download this dictionary for you if you don't
 # have it already.
 
 path = ARGV.length > 0 ? ARGV[0] : '/usr/share/dict/words'
-entries = File.read(path).split("\n")
+entries = File.read(path).split("\n").to_set
 
 contents = $stdin.read
 output = contents.gsub(/[^ \n]+/) do |word|