You can get 2 or more void upgrades in the same drawer. It should limit to 1 void upgrade per drawer

Modpack skyfactory 3

Should limit redstone upgrades too, if they aren't already.

Other than being silly and occupying extra slots, are there any further consequences?

I guess it only comes up when you're applying a lot of void upgrades to a lot of drawers.... which would happen in Skyfactory when you want to void all of the extras in a system that makes 20 different items from crushed cobblestone.

Fixed in 3.6.0. Only normal storage upgrades can be duplicated, and only one upgrade from each "class" can be used at a time.