bench.rb

require "benchmark"

class Array
  def duplicates_reject_uniq
    reject { |e| count(e) <= 1 }.uniq
  end

  def duplicates_uniq_reject
    uniq.reject { |e| count(e) <= 1 }
  end
end

n = 1000
Benchmark.bm do |bm|
  bm.report("reject then uniq") { n.times { ([1] * 100).duplicates_reject_uniq } }
  bm.report("uniq then reject") { n.times { ([1] * 100).duplicates_uniq_reject } }
end

#       user     system      total        real
# reject then uniq  0.070000   0.000000   0.070000 (  0.078661)
# uniq then reject  0.020000   0.000000   0.020000 (  0.013102)