Here is what happens in JS console (+warning in UI) when doing the ${subject}:

POST http://localhost:8080/hawkular/alerts/triggers 400 (Bad Request)
POST - Trigger with ID [f52adbe73fbac9ded8cafc5cfcae67a1_trigger_thres] exists.

It happens because of this lines https://github.com/hawkular/hawkular/blob/b318df5def29f0f4e383ed8b8160b225043ada36/ui/console/src/main/scripts/plugins/metrics/plugins/metrics/ts/addUrlPage.ts#L151

The trigger name should contain also the persona id to be unique (MD5 hash of the URL is not enough)