Change #1306121 had a related patch set uploaded (by Zaidusyy; author: Zaidusyy):

[mediawiki/extensions/ContactPage@master] T430432: Display contact confirmation in success box

https://gerrit.wikimedia.org/r/1306121

Ideally, after submitting, this button would become deactivated, along with providing some confirmation that the submission occurred:

Having a confirmation without deactivating the button isn't going to fix the underlying issue - this form gets repeatedly submitted by users who just keep clicking the submit button.

I've uploaded a new patchset that implements a client-side JavaScript module to disable the submit button asynchronously upon submission. I also wrapped the confirmation message in Html::successBox() for better user feedback.

At the page https://meta.wikimedia.org/wiki/Special:Contact/Stewards, @Xaosflux observes that many users are clicking the send button multiple times ...

That's probably because of T383047

because the screen does not transition to a completed state after the send button is clicked.

After user submits, the form gets replaced with a success message: https://www.mediawiki.org/wiki/MediaWiki:Emailsenttext.

But because of the mentioned bug now, the form gets server rerendered (for error display) and that's why people resubmit (they've no way of knowing the first submission worked).