Adding a hash to a DM collection makes two items

dm_bug.rb

require 'dm-core'
DataMapper.setup(:default, 'in_memory::')

class A
  include DataMapper::Resource
  property :id, Serial
  property :jada, String
  has n, :bs
end

class B
  include DataMapper::Resource
  property :id, Serial
  property :joda, String
end

DataMapper.finalize

a = A.create
a.bs << {:joda => 'test'}
a.save
p a.bs

#=> [#<B @id=1 @joda="test" @a_id=1>, #<B @id=1 @joda="test" @a_id=1>]