fix duplicate unretired communication channels

fixes CNVS-13030

Change-Id: I3a9fc310645aff71c677060fcfe2b0b7983769cf
Reviewed-on: https://gerrit.instructure.com/34975
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>