array__unique_by_attr.rb

arr = [{:email => 1, :name => "foo"}, {:email => 1, :name => "bar"}, {:email => 2, :name => "foo"}]

uniqueness_by = :email

result = arr.inject({:collected => [], :indicators => []}) do |helper,item|
  indicator = item[uniqueness_by] # indicates uniqueness
  unless helper[:indicators].include?(indicator) # new indicator? then collect it!
    helper[:indicators] << indicator # for later decisions
    helper[:collected] << item # the actuall result to the user
  end
  helper
end

unique_arr = result[:collected] # [{:email => 1, :name => "foo"}, {:email => 2, :name => "foo"}]