GitLab CI lint could warn when multiple jobs in a pipeline deploy to the same environment

Summary

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/57820#note_177234423 - if you define a .gitlab-ci.yml that has multiple jobs that deploy to the same environment it can result in confusion (as it did for me).

It would probably be handy if the CI lint or gitlab itself raised a warning about the duplicate environment tag, then I might've understood what was going on more easily!