How to do cell counts for a Z stack?

I have a set of tif files from a confocal microscope that produced a Z stack for a embryo, I'm trying to count the total number of cells in this Z-stack. Does anyone know how I can exclude cells already counted in previous images?

ie. the same cell may appear in more than one image since the cells have depth, how do I make sure I only count these cells once?

Right now, I have code to analyze one image and count all the cells in one image already.

Thanks!