If double stack it's toxic for melee, single stack is toxic for ranged: A melee can be kited easily, so a ranged (even against a jax or a xin) will have at least 2/3 hits on you before you can actually attack them and you also have to waste a gap closer do reach them usually (and against a vayne or a tristana it's pure bs, they're untouchable) so it's just not fair for melee this way.
If you think about it, with how things are right now a ranged can easily stack on a melee those 2/3 hits, so when the melee gets on top of you he has to hit you 6 times, while you only needs another 3 hits, but if we give the double stack to melee, you will both stack it pretty much at the same time. Also the toxic behaviour wasn't the double stacking itself, but the strong sinergy with devourer, that let melee charge rageblade in 3 hits while ranged had to do the full 8 hits (I don't remember if runaan worked with the stacks though), giving melee a HUGE advantage over ranged. Now that devourer is no more in the game, the double stack should be re-implemented since it would just even the field, it's just not right that if I fight a vayne, by the time I reach her, she has 3 stacks, then she condemns me and I still didn't hit her once so if I manage to reach her again I will need to stack it from 0 stacks while she will have full rageblade already and, if she could, she's probably killed me 4 times before I actually land my first hit.