showInNavigator duplicates IDs of series

[pawelfus]

Expected behaviour

IDs should be unique for series, even when it's a part of a navigator.

Live demo with steps to reproduce

http://jsfiddle.net/rdqh1rad/

In the console, we can see 3x highcharts-navigator-series. It will become a problem, when we would like to use chart.get('highcharts-navigator-series');