Please decipher the following error message: "Multiple keys in #<KeywordUser @user_id=1 @keyword_id=2>"

Question

jruby google app engine error message: Multiple keys in #<KeywordUser @user_id=1 @keyword_id=2>

Models: (attempting to implement has and belongs to many relationship)

class Keyword
  include DataMapper::Resource
  property :id, Serial # required for DataMapper
  property :name,      String   # A varchar type string, for short strings
  has n, :users, :through => Resource
end


class User
  include DataMapper::Resource
  property :id, Serial # required for DataMapper
  has n, :keywords, :through => Resource
end

(in the code)
newkeys.each_with_index do |key,index|
  if Keyword.first(:name => key)
    self.keywords << Keyword.first(:name => key) unless self.keywords.first(:name => key)
  else 
    new_keyword = Keyword.create(:name => key)
    new_keyword.save
    self.keywords << new_keyword
    self.save
  end
end
self.save # error happens here