You can disagree all you like, but you're wrong.
Russia is a dictatorship. The Duma is permitted to exist to give Russians, and credulous fools abroad, the illusion of democracy and help reinforce the idea that Putin represents the will of the people. (In a sense he sort of might truly represent the will of the people, if only because he's destroyed independent media and propagandised enough of them into compliance.)
That's what pretty much every dictator does: leave a parliament around to pretend there's still some independent power and popular representation, or just to provide a pool of people to act as government ministers. It's not even a new idea: even all the way back in the Roman Empire, once the Republic was dissolved the emperors left the Senate as a facade that Roman governance was anything other than the emperor's brute force.
At best, like many dictators Putin has to maintain sufficient favour, otherwise someone might push him off a tall building. Thus the Duma could in theory oppose Putin, if Putin's control of the country slipped far enough. There's no sign that Russia is close to that point.
Well, this is true. Currently, with a huge numerical advantage, Russia is taking massive casualties to grind the frontline forward a kilometre or two a month. In 2022, with a huge numerical advantage, it was taking massive casualties and losing swathes of territory. In three years, it has improved to the point it can grind down a vastly weaker opponent through attrition.