A trade up sort of system where you take 5 or more cards of the same rarity to make a new one of better rarity works only if your currency is very difficult to inflate, or based on IRL currency such as CS:GO. However getting rid of old cards to gain boosts for said card could work instead of gold without hurting much economy. But it won't get rid of a lot of duplicates.
To stop multiaccounting you should totaly avoid daily rewards and perhaps try to make a small Bfp reward for each PvP or PvE game that you won, increasing per difficulty of the match. For example the first maps (the first chain of maps): -standard mode no rewards -advance mode 1-2 bfp -expert mode 3 bfp The second chain of maps -standard 1 bfp -advance 2-3 bfp -expert 4 bfp and so on...