duplicate SQL insert on has_many through

I'm getting a strange case where saving an object that has a has_many/through does a double-SQL-insert on the join table. Unforunately, I can't get it to happen with a simple example (yet).

The left side of the relationshp is new. The right side is either new or not; doesn't seem to matter. The join table has a unique constraint, so an SQL error is thrown.

It runs the validations, the saves, and the after saves ... then it seems to run something again. This doesn't happen in the simple case, so in that case, it works. It looks from the stack trace like it's trying to run update on something (?) and thinks something is new still, a la @new_record_before_save in autosave_association.

Here's the stacktrace against 2-3-stable, but it happens against master and 2-2-stable as well.

.../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb:212:in `log': Mysql::Error: Duplicate entry '1230-40' for key 2: INSERT INTO `taggings` (`created_at`, `video_id`, `tag_id`, `updated_at`) VALUES('2009-04-19 18:53:19', 1230, 40, '2009-04-19 18:53:19') (ActiveRecord::StatementInvalid)
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb:320:in `execute'
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:259:in `insert_sql'
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb:330:in `insert_sql'
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:44:in `insert_without_query_dirty'
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/query_cache.rb:18:in `insert'
	from .../vendor/rails/activerecord/lib/active_record/base.rb:2902:in `create_without_timestamps'
	from .../vendor/rails/activerecord/lib/active_record/timestamp.rb:53:in `create_without_callbacks'
	from .../vendor/rails/activerecord/lib/active_record/callbacks.rb:266:in `create'
	from .../vendor/rails/activerecord/lib/active_record/base.rb:2868:in `create_or_update_without_callbacks'
	from .../vendor/rails/activerecord/lib/active_record/callbacks.rb:250:in `create_or_update'
	from .../vendor/rails/activerecord/lib/active_record/base.rb:2556:in `save_without_validation!'
	from .../vendor/rails/activerecord/lib/active_record/validations.rb:1019:in `save_without_dirty!'
	from .../vendor/rails/activerecord/lib/active_record/dirty.rb:87:in `save_without_transactions!'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:200:in `save!'
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:136:in `transaction'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:182:in `transaction'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:200:in `save!'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:208:in `rollback_active_record_state!'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:200:in `save!'
	from .../vendor/rails/activerecord/lib/active_record/validations.rb:990:in `create!'
	from .../vendor/rails/activerecord/lib/active_record/reflection.rb:179:in `create_association!'
	from .../vendor/rails/activerecord/lib/active_record/associations/has_many_through_association.rb:60:in `insert_record'
	from .../vendor/rails/activerecord/lib/active_record/base.rb:2148:in `with_scope'
	from .../vendor/rails/activerecord/lib/active_record/associations/has_many_through_association.rb:60:in `send'
	from .../vendor/rails/activerecord/lib/active_record/associations/has_many_through_association.rb:60:in `insert_record'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:145:in `send'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:145:in `send'
	from .../vendor/rails/activerecord/lib/active_record/autosave_association.rb:294:in `save_collection_association'
	from .../vendor/plugins/will_paginate/lib/will_paginate/finder.rb:168:in `method_missing'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:368:in `method_missing_without_paginate'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:219:in `method_missing'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:219:in `each'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:219:in `send'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:219:in `method_missing'
	from .../vendor/rails/activerecord/lib/active_record/associations/association_collection.rb:368:in `method_missing_without_paginate'
	from .../vendor/plugins/will_paginate/lib/will_paginate/finder.rb:168:in `method_missing'
	from .../vendor/rails/activerecord/lib/active_record/autosave_association.rb:286:in `save_collection_association'
	from .../vendor/rails/activerecord/lib/active_record/autosave_association.rb:169:in `autosave_associated_records_for_tags'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:181:in `evaluate_method'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:181:in `evaluate_method'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:181:in `instance_eval'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:181:in `evaluate_method'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:166:in `call'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:93:in `run'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:92:in `each'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:92:in `send'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:92:in `run'
	from .../vendor/rails/activesupport/lib/active_support/callbacks.rb:277:in `run_callbacks'
	from .../vendor/rails/activerecord/lib/active_record/callbacks.rb:344:in `callback'
	from .../vendor/rails/activerecord/lib/active_record/callbacks.rb:283:in `update'
	from .../vendor/rails/activerecord/lib/active_record/base.rb:2868:in `create_or_update_without_callbacks'
	from .../vendor/rails/activerecord/lib/active_record/callbacks.rb:250:in `create_or_update'
	from .../vendor/rails/activerecord/lib/active_record/base.rb:2539:in `save_without_validation'
	from .../vendor/rails/activerecord/lib/active_record/validations.rb:1009:in `save_without_dirty'
	from .../vendor/rails/activerecord/lib/active_record/dirty.rb:79:in `save_without_transactions'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:229:in `send'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:229:in `with_transaction_returning_status'
	from .../vendor/rails/activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb:136:in `transaction'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:182:in `transaction'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:228:in `with_transaction_returning_status'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:196:in `save'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:208:in `rollback_active_record_state!'
	from .../vendor/rails/activerecord/lib/active_record/transactions.rb:196:in `save'
	from ../../../../bin/../lib/.../import/map.rb:128:in `import'
	from .../vendor/gems/fastercsv-1.4.0/lib/faster_csv.rb:1514:in `each'
	from ../../../../bin/../lib/.../import/map.rb:38:in `import'
	from ../../../../bin/../lib/.../import.rb:52:in `import'
	from ../../../../bin/import:15