please help

I have a series of 500 points and a grid (10x8). Both the number and location of the points and the number axes and spacing of the grid are variable.
The points are aligned in x and y with the grid, but their z coordinate varies. Sometimes several points are on the same grid point but at different heights. They form vertical stacks (they share their x and y coordinates with one of the grid points but have a different z coordinate).

I want to check the z-distance between the points within one stack and get rid of one of them if they are too close to each other. (that shouldnt be too difficult once I have identified the ones which are too close to each other.

Problem:

The z-distance to points in other stacks is not relevant. If I analyse the z-distance between all points I have to use cross reference and I get an enormous list of booleans, which kills my computer and is 99.9% irrelevant and I can not apply it to sort out my list of 500 points because booleans and points dont correspond.

It seems obvious to only measure between the points within each stack - but how? It seems very obvious but how can I tell grasshopper to take all the points out of a series which have identical x and y coordinates to check the differences between their z coordinates?

Also remember that the grid and therefore the number of stacks needs to stay variable and can go into the hundreds - so seperating branches using 'explode tree' or 'tree branch' doesnt work.

I have spent hours and hours trying, but cant get it t work - any suggestion would be appreciated!