You can't convert the card when you get the card for the first time. However, if you get the exact same card again (same card, same rarity), it will automatically increase 1 level of that card.

For example, if you orginally have a Lv 2 Common Elenna tower card and now you got one duplicate, your Common Elenna will become Lv 3 instead.

And if your duplicate card already reached max level, we will reward you with Gems when you get the card from Premium Battle Chest/ Hero Chest.

For Tower/Spell card:
Common = 25 Gems
Rare = 50 Gems
Epic = 100 Gems
Legendary = 150 Gems

For Hero card:
Common = 40 Gems
Rare = 80 Gems
Epic = 160 Gems
Legendary = 240 Gems

And if you get the card from non gem-related circumstances, we will reward you with Coins instead:

Common Card = 1000 Coins
Rare Card = 2000 Coins
Epic Card = 3000 Coins
Legendary Card = 4000 Coins