There are two variations the devs can do of it when there are multiple items of the same type:

1. Show only the tooltip of the first of them, the actualy place the item will go when you equip it. This will provide less info about your items, because you still don't see your other items.

2. Show the tooltip of all of them. This can occupy the whole screen and also can cause mixed tooltips, especially when you hover over a module or kit with a mech that can equip a lot of these. This would require special coding or it will confuse the player instead of helping him(or her)...
My two cents about it...

Can we have it as it is now, with the addition that pressing (say) control will pop up the comparison tip between the viewed item and the currently equiped item in the same slot (as the viewed item would be equiped).
This would avoid filling the screen with tips each time the mouse drifts over something, but provide very useful comparison to the less experienced.

So I guess thats a 'tweak' vote, although I didnt submit a vote yet.

It's true for kits, but for modules, you can equip any amount of any type of modules except the resistance modules, you are limited to equip only one module for each resistance type. Multi resistance modules increase all of the resistances, so it counts for all three of them thus preventing you from equipping any other resistance modules.(The same goes for the kits)

I have actually suggested this a long time ago when I first started playing sm.

Bumpity-Bump-Bump...