I don’t have full information on the causes of this, but sometimes Echo notifications render twice for me: one popup is in top left (and it stays permanently until you click away), the other − under the icon (shown while you are on top of the page). I’ve seen this twice already, but I can’t reproduce it after it happens.
As you can see, this is not specific to one type of alerts.
Browser information: Firefox 57, Windows 8.1, touch enabled.