Combining Duplicates
When you pull duplicate cards, you can combine them to increase the card's level, which directly boosts its stats (Pace, Shooting, Passing, Dribbling, Defense) and its passive cash generation.
Upgrade Strategies
Only upgrade cards that you plan to keep on your plot or in your Bank. Do not waste duplicates of low-tier cards on upgrades; sacrifice them in the Craft Shop for Trophies instead.