DA - get only unique entries from sql view

fixes CNVS-15063

test plan:
  - in course with DA
  - as a student submit multiple times to a selrel'd assignment
  - go to grade summary page and ensure there aren't dupes
  - in a course without DA, everything still works

Change-Id: I7bd24b71bc8661f9747200579e7c11ade4fac1bf
Reviewed-on: https://gerrit.instructure.com/39894
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>