Semgrep should scan only languages not covered by Advanced SAST, even under Policy
The SAST template is defined so that Semgrep will scan only languages not covered by GLAS.
See: /lib/gitlab/ci/templates/Jobs/SAST.gitlab-ci.yml#L182
Thanks to @odupre, we discovered an edge case. When SAST scanners are run using Policies, both Semgrep and GitLab Advanced SAST are running and they are scanning the same files, leading potentially to duplicate/redundant results.