don't include users multiple times in report

fixes CNVS-10382

test plan
 - enroll user in multiple courses or multiple times
 - run last_user_access report
 - user should be included once per pseudonym

Change-Id: I82673fee4780da10b713b20c8cf7ee197928d54d
Reviewed-on: https://gerrit.instructure.com/28402
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>