Stacked spell overridden by non stacked

When I stack several "ignite weapon" spells then cast it... if I then cast a single ignite weapon spell it over rides the first cast spell reducing my extra damage from the stacked cast.

I think it should retain the stronger cast spell if I make the mistake of casting a lower version afterwards.

I'd like to see casting a single spell refresh timer for whatever level of stacked version I presently have up.