Steps to reproduce:

1. Set browser.urlbar.update2.engineAliasRefresh = true to enable the new add search engine dialog from bug 1106626
2. Go to about:preferences#search
3. Click Add button under One-Click Search Engines
4. Add an engine with any name and URL and the alias "test"
5. Add a second engine with a different name but the same alias "test"

Actual results:

Search engine with a duplicate alias is added.

Expected results:

The Add Engine button should be disabled with a validation message "An engine with that alias already exists."

I think this was a copy and paste error at https://searchfox.org/mozilla-central/rev/ce21a13035623c1d349980057d09000e70669802/browser/components/preferences/dialogs/addEngine.js#56

Should be
validity = document.getElementById("engineAliasExists").textContent;

Yup, doing a last minute change before pushing is always a bad idea

The severity field is not set for this bug.
:daleharvey, could you have a look please?

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Fixed this in the dupe