Select Names with Matching Last Name and First Initial from a list

names.rb

require 'csv'

CSV_FILENAME = "some_names.csv"

names_hash = {}
matches_array = []

CSV.foreach(CSV_FILENAME) do |row|
  last_name,first_name = row[0],row[1]

  if names_hash[last_name].nil?
    names_hash[last_name] = []
  end
  names_hash[last_name].push(first_name)
end

names_hash.map do |last_name, first_names|
  initials = first_names.map {|name| name[0]}.uniq
  initials.each do |initial|
    matching_names = first_names.select {|n| n[0] == initial}
    if matching_names.count > 1 
      matching_names.each do |first_name|
        matches_array << "#{first_name} #{last_name}"
      end
    end
  end
end

puts matches_array