The idea of a duel-kit champion is that they shouldn't be any stronger than one with 4 abilties

Why is this not true in any case though?

Each duel-kit champion has the following-

Hard engage Hard disengage Sustained damage Burst damage % damage of some sort An attack speed boost from an ability or passive A movement speed boost of some sort Long-mid range harass

Which means that in any given scenario, a duel-kit champion has COMPLETE control over who dies and who doesn't. They don't have to engage if they don't want to, and cooldowns are no longer a weakness, having burst damage AND sustained damage means they can burst you down to low health, yet still have abilities to back themselves up if you decide to go in.

They are virtually untouchable in a gank, and have some form of dealing a secondary form of damage should you decide to build a resistance to one.

How is this Ok? There is no downside to having a duel-kit, in fact there is no reason NOT to have an ultimate, having all this you are in complete control over who dies and who doesn't.

The idea of a duel-kit is that you should have more at your disposal, but in return they should be half as strong, or at least quite a bit weaker than those who don't. You have a wider variety, and can deal some damage while still having abilities up if a time should arise, but making that each ability is just as potent or MORE than a regular kit, while still having low cooldowns and all the advantages of a normal kit is just, well, broken.

A duel-kit is perfection, and it's not fair to those who don't have those to dispose of.

And a side note, don't tell me to ban them, that's already a weak argument, but there are still more duel-kit champions than I have bans, and that's IF I get first pick, and that's IF I don't want to ban any normal kit equipped champions.