Why are these named_scopes causing a duplicate INNER JOIN?

I have a Model which I am using to track permissions in a hierarchical organization using the awesome_nested_set plugin. I'm running into a problem where two named_scopes, when chained together, are creating a duplication INNER JOIN.

class Group < ActiveRecord::Base
  acts_as_nested_set
  has_many :memberships
  has_many :accounts, :through => :memberships
  has_many :authorizations
  has_many :users, :through => :authorizations
end

The two named_scopes are located in the Account model, and are used to filter accounts by a user and by a group:

named_scope :in_group, lambda { |id|
  group_ids = Gr开发者_开发技巧oup.find(id).self_and_descendants.collect(&:id)
  { :joins => :memberships, :conditions => ["memberships.group_id in (?)", group_ids] }
}

named_scope :for, lambda { |user|
  groups = user.authorizations.groups.collect(&:id) unless user.admin?
  user.admin ? {} : { :joins => :groups, :conditions => ["groups.id IN (?)", groups] }
}

Both of these named_scopes need to join memberships, but shouldn't they be able to do that without duplication? When they are chained together, mysql blows up with the following error:

Mysql::Error: Not unique table/alias: 'memberships': SELECT `accounts`.* FROM `accounts` INNER JOIN `memberships` ON accounts.id = memberships.account_id INNER JOIN `memberships` ON `accounts`.`id` = `memberships`.`account_id` INNER JOIN `groups` ON `groups`.`id` = `memberships`.`group_id` WHERE ((memberships.group_id in (54,94)) AND (groups.id IN (54,94)))  ORDER BY business_name, location_name LIMIT 0, 75

Try changing the join statement in the for named scope to :joins => {:memberships => :groups}. I suspect that the has_many through relationship might be causing it.