has_many through append to collection should have duplicates before reload

There seems to be a problem with the behavior of has_many through.

When you access the collection before appending multiple identical objects, the collection is not updated. For example the following RSpec should pass:

Models
Package has_many categories through package_categories
PackageCategory belongs to package and category
Category has_many packages through package_categories

THIS FAILS (on the last line):

@p = Package.create
@cat1 = Category.first
@p.categories.map(&:id).count.should be 0
@p.categories << @cat1
@p.categories << @cat1
@p.categories << @cat1
@p.categories.map(&:id).count.should be 3

THIS PASSES:

@p = Package.create
@cat1 = Category.first
@p.categories << @cat1
@p.categories << @cat1
@p.categories << @cat1
@p.categories.map(&:id).count.should be 3

THIS PASSES:

@p = Package.create
@cat1 = Category.find(1)
@cat2 = Category.find(2)
@cat3 = Category.find(3)
@p.categories.map(&:id).count.should be 0
@p.categories << @cat1
@p.categories << @cat2
@p.categories << @cat3
@p.categories.map(&:id).count.should be 3

It seems to be isolated to added multiple of the same object after previously accessing the collection.

Any ideas?