Problem with has_many :through, :uniq => true with polymorph

Didn't have quite enough space to describe it there...basically i'm
having a problem with the :uniq option in my tags.

I'm using acts_as_taggable_on_steroid which adds these associations to
my Resource class:

Resource
  has_many :taggings, :as => :taggable, :dependent => :destroy, :include
=> :tag
  has_many :tags, :through => :taggings, :uniq => true

This uses the join model Tagging:

Tagging
  belongs_to :tag
  belongs_to :taggable, :polymorphic => true

And of course the Tag model:

Tag
  has_many :taggings, :dependent => :destroy
  has_many_polymorphs :taggables,
    :from => [:resources, :lessons],
    :through => :taggings,
    :dependent => :destroy

The uniq option doesn't seem to stop me from adding the same tag
multiple times to a resource:

resource = Resource.first

=> #<TeachingObject id: 59, name: "Clarinet bite point close-up" ...>

tag = Tag.first

=> #<Tag id: 1, name: "recorder">

resource.tags

=> [#<Tag id: 1, name: "recorder">, #<Tag id: 610, name: "chunky
chicken">, #<Tag id: 1, name: "recorder">]

resource.tags << tag

=> [#<Tag id: 1, name: "recorder">, #<Tag id: 610, name: "chunky
chicken">, #<Tag id: 1, name: "recorder">, #<Tag id: 1, name:
"recorder">]

resource.tags << tag

=> [#<Tag id: 1, name: "recorder">, #<Tag id: 610, name: "chunky
chicken">, #<Tag id: 1, name: "recorder">, #<Tag id: 1, name:
"recorder">, #<Tag id: 1, name: "recorder">]

Now, i have other classes which use has_many :through with :uniq => true
and they seem fine: for example. resources also join to assets, through
a table called 'items':

Resource
  has_many :items, :dependent => :destroy, :order => :position
  has_many :assets, :through => :items, :order => :position, :uniq =>
true

Item
  belongs_to :asset
  belongs_to :resource

Asset
  has_many :items, :dependent => :destroy, :order => :position
  has_many :resources, :through => :items, :order => :position

Like i say, this behaves as i'd expect:

resource = Resource.first

=> #<TeachingObject id: 59, name: "Clarinet bite point close-up" ... >

asset = Asset.first

=> #<Asset id: 1, name: "07 Clari Clari (notation, perf AI)" ... >

resource.assets

=> []

resource.assets << asset

=> [#<Asset id: 1, name: "07 Clari Clari (notation, perf AI)" ... >]

resource.assets << asset

=> [#<Asset id: 1, name: "07 Clari Clari (notation, perf AI)" ... >]

See? It doesn't add the duplicate.

I'm wondering if it's to do with the polymorphic nature of the tag -
resource relationship - maybe the polymorphism is stopping it from
noticing that it's not uniq? or something?

Grateful for any help or ideas...i can't even think of a way to work
around this since the problem is buried inside active record's <<
method.