これが解決策です。複数のフィールドでクラスを再利用する場合は、クラス名を指定できます。
class Email
include Mongoid::Document
embeds_many :to_recipients, :class_name => "Recipient"
embeds_many :cc_recipients, :class_name => "Recipient"
embeds_many :bcc_recipients, :class_name => "Recipient"
embeds_one :from, :class_name => "Recipient"
field :subject, type: String
field :body_text, type: String
field :body_html, type: String
end
class Recipient
include Mongoid::Document
field :email_address, type: String
field :name, type: String
validates :email_address, :presence => true
embedded_in :emails
end