In Star Wars Battlefront 2, Star Cards power-up your character on the field:
Can you use two Star Cards with the same name at the same time? Do the abilities stack?
You can only own one copy of a Star Card with a given name at a time, per class, and it is of the highest rarity. All other instances of that care you obtain are counted as duplicates and cashed out for credits.
For instance, I own the Enhanced Thermal Detonator (Epic, so ****) Star card. Out of a loot crate, I received the same Star card - "Enhanced Thermal Detonator" - as a common (*), which was promptly marked as a duplicate by the system as turned into 200 credits. Likewise for Uncommons, which become 400 credits, and Rares which become 800 credits.
Do you have a specific, first hand example of owning two Star Cards for the same class, with the same name? In my experience this should be impossible.
I am unsure what happens when you have a lower rarity of a card and obtain a higher rarity of the same card out of a crate - I would assume it is simply replaced and you get no credit benefit, or maybe the game is generous (doubtful) and refunds you a few credits.