Emails on Push submits "Auto-Submitted" header twice

The fact that the header is added twice leads Amavis-New to classify the mail as SPAM, because no header should be repeated and very often malicious software fails to generate properly formatted e-mail.

Since GitLab 9.0.1, GitLab's Email on Push functionality falls into that category, unfortunately...

If I were able to find where the duplicate comes from, I'd file a PR instead of an issue, but I could only find 1 occurrance of 'Auto-Submitted' in the actual code base.

The headers on the rejected e-mail look like this:

X-GitLab-Project: cmucl
X-GitLab-Project-Id: 202
X-GitLab-Project-Path: cmucl/cmucl
X-GitLab-Author: root
Auto-Submitted: auto-generated
X-Auto-Response-Suppress: All
Auto-Submitted: auto-generated
X-Auto-Response-Suppress: All