Live demo at http://jsfiddle.net/highcharts/fq4rxp31/, run http://jsfiddle.net/highcharts/fq4rxp31/show on iOS.
In Safari on iOS 8.1.1 (tested on iPhone 6), when a second text-shadow is applied on and SVG text element, a duplication of the text element is rendered below it. It does not happen in desktop Safari or Chrome. It does however happen in PhantomJS.
Probably related, but not identical to #111216
Just found out it only applies to Retina displays. On my Mac, if I have an external monitor with 1:1 display, I can drag the browser to the Retina display and back to the external monitor and see the artefact appear and disappear.