Responses to teacher msgs. show twice in pending queue

Here are the steps:

1) Teacher writes message in student mailbox
2) Student reads message and clicks on msg. to respond to it (making a threaded response)
3) In teacher pending queue, there now appears:

    a. Teacher msg. with button "Mark as Reveiwed"
    b. Student response with button "Mark as Reviewed"
    c. Student response as unthreaded msg. with "Mark as Reviewed"

Only a and b should appear, and a should not have button "Mark as Reviewed".