The correct spelling is simply "Kersten", without any Umlaut.
Gustav Kersten was a gunmaker in Strassburg (Strasbourg, in the Alsace), part of the german empire before the 1. war.
In Germany this is also known as "double Greener", meaning two ears, one on each side.

For what I know, there is no "double Kersten".

Regards,
fuhrmann