Do not maxed out your 5* champ
When you have a maximum level 4* (Lvl 99) and you duped it again, you get 275 5* shards, gold, ISO and a max signature ability crystal. The max signature ability crystal has a high probability of giving you another 275 5* shards.
I have a Lvl 200 5* Rogue that I duped again from the purchase of several GM crystals. Guess what? I only have 275 6* shards, gold and ISO. No max signature ability crystal. No other rewards. Seems that the rewards for duping a maxed out 5* is inferior to duping a non-maxed out 5*.
So don’t max out the 5*. In fact, not having a max signature ability crystal means there is no incentive to max out your 5*.