Detect upstream instances of the same effect.

Is it possible to detect only the upstream instances of the same effect? I am trying to make a displacement effect that uses a noise for randomized displacement and I want it to accumulate the displacement of the other instances and use it in the calculations. Basically:

First instance --> Displaces layer based on original X/Y(unaffected by its own displacement)

Second instance -> Detects the displacement the first instance produced and adds it to X/Y which will then create a displacement that moves the layer in a completely different direction than the first instance. (this instance must only be affected by the displacement of the first instance)

It is easy to detect all the instances, but I can't find a way to make the last instance not be affected by its own displacement. Basically all the instances start with the same X/Y and only add to them the displacement created by the upstream instances. Is there any way to do that? I need to find a way to tell the last instance say: "Hey! I am the last instance, I should not add my own displacement to X/Y"