The main issue I see with adding it is how to show off the info to the player, but i can think of two different possible implementations:
1. Having the modules stack ontop of eachother, similar to belt stacking. I personally prefer this implementation and think it looks best, but obviously its up to the discretion of whoever implements it

A couple values that I could see being useful for this is an extra parameter for icons_positioning, something like stacked_offset which'd specify the offset of every module thats stacked ontop of it; another being a utility constant (similar to belt stacking) which'd specify the amount of modules that can be rendered before it gets cut off. 2. Having a number next to the module, specifying how many is in that slot. This would be a lot less intrusive and would allow for a lot more modules to be in a single spot without it blocking the entire entity they have been placed inside; however i could see it being a little hard to read from afar compared to the first idea (but im not sure how much of a nuisance that would be? considering you'd probably be zoomed in anyway when configuring modules?) there are probably other ways to show off this information to the player, but i hope these ideas suffice :]