At the ship design screen of Vodyani Goaym class exploration ship with one weapon slot when you double click on a module to add to the slot you can add two modules to one slot. I also noticed vision ranges also add up so, for example you can create a ship with 2 hyperium engines and 4 hyperium probes with 2 slugs with a huge vision range. I don't think it is intentional because it doesn't work when you drag and drop a module to a slot.

Oh sorry I didn't see your message. I can create super exploration ships with this bug, they just go on starlanes super fast and find all hidden systems around with huge vision range, with 6 titanium probes get all the curiosities in a system in one turn and they replenish next turn so you can probe everywhere with your infinite probes. I feel guilty to use it but it is just too good. Well, i think it really breaks the balance, it would be good if it gets fixed.