edx/edx-platform — pull request #1351 by brianhw (2013-10-15 18:25:42+00:00)
Title: Check that email subtasks are known to the InstructorTask before executing.

Addresses a problem where bulk email subtasks are duplicated at the point that a group of subtasks is queued. Those with a requested subtask_id are created correctly, and duplicate subtasks with different ids are also created with otherwise identical arguments. This fix prevents the duplicate subtasks from actually running (and sending duplicate emails).