How to stop callbacks from getting called twice

Maybe this is old news, but it took me all afternoon:

In my test environment, an 'after_create' callback was getting called
twice unexpectedly. In my model, I had a simple 'after_create :foo'
declaration.

From what I understand, the problem seems to be that the 'after_create
:foo, :bar' syntax appends those symbols to an array without doing a
'uniq'. It seems that if you happen to load the class twice, you'll get
double entries in the callback list. This can happen easily because the
Ruby 'require' method doesn't seem to convert paths to absolute paths,
e.g the following will cause the file '/lib/foo.rb' to be loaded twice:

# In \app\models\bar.rb
require File.join(RAILS_ROOT, 'lib', 'foo')
require '../../lib/foo'
class Bar < ActiveRecord::Base

end

So, I added this to my environment.rb:

module Callbacks
  def callbacks_for_with_uniq(method)
    callbacks_for_without_uniq(method).uniq
  end

  alias_method_chain :callbacks_for, :uniq
end

- OR -

You could use the alternate form of defining a callback, but this is
less convenient.

def after_create
  # ...
end