Let's imagine a scenario were the player doesn't want to spend fate points on a combat, so he want to stack several free invocations to go for a big hit in a conflict.
This conflict happens on a old house and has the situation aspect "Dust everywhere" the PC is fighting against a NPC.
The player wants to create a few freebies on that aspect. So the first turn he declares that he wants to jump and circle around the NPC to raise dust and obscure his vision. The next turn he wants to create and advantage again throwing a handful of dust to the NPC's face. And so on...
This is just an example, but the question remains.
Can you use create advantage on an existing aspect more than one time?
Can you stack free invocations?