Flamerstrike ticks

Can someone explain me how does flamestrike works ?

When I cast Flamerstrike (w/e range) it puts on ground fire, that deals damage over "x" seconds.
When I cast Flamestrike once again in this same place it ticks only once. It looks like I used flamerstike only once, but I used it twice.

Why ?

This is how it should work :D?

Yep, this is how it's meant to work. What you can stack is the periodic damage from preceding ranks of Flame Strike.