Here is the problem, even P2P need to spend to get duplicate non valiants as they are sometimes more useful than the valiant counterpart, if you make this to happen they will spend way less, just look at the arena right now, there is not full valiant team there. When the F2P reach them with this system they will feel the need to summon for valiants, may it be with Blood Tokens or spending a lot more money to get them, now we will start to lose some of them, when they realize that chasing valiants will be a huge money sink. Not every spender in the game is a whale, not every game survives only on whales. The fast we stay on toe with the P2P players the more the devs will need to make things to appeal to the heavy spenders, broken, better units, where the strategy will go? and more the game will be broken because of that. Even than we have many other limited resources with fodders, gold and job keys.When F2P have caught in in terms of equipments, P2W will be ahead in levels for both valiants and non-valiants. With my suggestion, F2P will be able to catch up in terms of levels for the non-valiants at least. P2W will always be ahead, but F2P will be allowed to catch up. Then comes the point where both P2W and F2P have maxed leveled non-valiants, but P2W will have the choice of using their highly-leveled (might not be maxed) valiants to give them the edge. In the future, more powerful units (like the lords and emperors in 7Knights) might be released, once again driving P2W players to spend again.The major gap between f2p and p2p now are the equipments. We Will get there now that we can enchance our equips, sure the bonus won't be the same but it Will certain help. F2p will get closer and closer with time. P2p needs to feel a least a little more powerful to keep spending.
At every point in time, P2W players will be motivated to keep spending, or smalltime paying players will keep up with them.
Imagine a world where they release a non valiant that can be even better as one and people will be stocking random spare 5* and in the moment this hero get released, everyone will be limit breaking him... the game can't be designed to be like that.
Look I'm a F2P and your suggestion would affect me, but even if it benefits me it will no benefit the game.