Add simple duplicate redirect finder

Resolves #1067 (closed) by adding a simple duplicate redirect finder to the CI. Only one line and uses busybox as the image, so it completes very quickly.

• I added three duplicates randomly in the redirect file (in a separate commit), and it properly fails and lists the duplicates: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/1501100479
• I removed the commit with the duplicates, and it runs green: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/1501104880 (in 9 SECONDS!)