uniq.rb

require 'benchmark/ips'

ARRAY = [1, 2, 3, 1]

def fast
  ARRAY.uniq!
end

def slow
  ARRAY.uniq.length != ARRAY.length
end

Benchmark.ips do |x|
  x.report('uniq!') { fast }
  x.report('uniq.length') { slow }
  x.compare!
end

__END__

Warming up --------------------------------------
               uniq!   666.486k i/100ms
         uniq.length   466.625k i/100ms
Calculating -------------------------------------
               uniq!      6.596M (± 2.9%) i/s -     33.324M in   5.056745s
         uniq.length      4.654M (± 1.5%) i/s -     23.331M in   5.014560s

Comparison:
               uniq!:  6596204.0 i/s
         uniq.length:  4653775.4 i/s - 1.42x  (± 0.00) slower

$ ruby -v
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]