Thunder Strike is one of those spells that should stack with each cast, instead of refreshing the duration.

The spell is supposed to do 4 (8 with talent) strikes when you cast it on a target. But when you recast it, the new cast is much less effective, because it only extends the duration of the previous cast, adding up to 3 strikes to it, missing out 1 damage instance.

Modifiers with timed effects, or a set amount of instances should stack, and not refresh on recast.