Talk:Shield Upgrade/@comment-12.157.17.210-20180124144259/@comment-454133-20180124193215

Technically you could have any non-limited, non-unique upgrade twice on one ship, and many ships can. If you have two missile slots, you can put Concussion Missiles in each slot and it's not a problem.

As mentioned, Modifications are a bit special, in that all ships have one modification slot and only a select few can equip more. Those that can always say "different" modifications.