[Solved]-why my label formatter is not working as expected in highcharts-Highcharts

Just formatter is called twice :) See JS console: http://jsfiddle.net/md2zk/66/ - each timestamp is listed twice.

As solution, I would clean up UsedLabels in tickPositioner:

$('#MessagePerformanceChartContainerID').data("UsedLabels", []);

Demo: http://jsfiddle.net/md2zk/67/