I think I understand your question (although it is a bit harder to understand the biological goal without more explanation!)
I think most likely there is not currently a module in CellProfiler that can do precisely what you request. I think you are requesting this: if multiple nuclei are within a certain distance of each other, arbitrarily (randomly) choose a single one of them for further analysis in the pipeline, without merging/fusing them together. Did I rephrase your question properly?
You can do something perhaps similar: First, use MeasureObjectNeighbors to measure the distance between each nucleus and its nearest neighbors. You then have two options: (1) FilterObjects to remove the nuclei that are < 20 pixels from a neighbor [to throw out ALL of nuclei that are near each other - not exactly what you want, I think] or (2) When you export your data to Excel, you can perhaps write a macro to see which nuclei have a neighbor < 20 pixels away, and then randomly choose which ones to throw out from your numerical analysis.
I don’t think a module to do exactly what you are asking is likely to be a very commonly used feature, so we won’t be adding it to CellProfiler, but I hope this is somewhat helpful.
Let me know if we can help further!