People have already reviewed and said everything there needs to be said so I will just outline what I liked or didn't like:

Liked:
1. The acting; The acting of the main leads especially wu jinyan was so stellar that in some parts I would forget that I am watching her act. She made sure that we would remember xue fangfei/jiang li. Her beauty and her expressiveness literally shined through the screen. The way she could deliver a thousand emotions through her eyes in a second was so good. Same goes for wang xingyue as duke su. Although he did not have much screen presence, he still managed to DELIVER. The moves that would have looked ridiculous and cringy with another actor, he made it work. Both of their chemistry was OFF THE CHARTS even when they are just looking at each other which was 90% responsible for this drama being a success. Shout out also to Shen Yurong and Princess wanning's actors because my goodness, they dominated almost every scene they were in. The way shen yurong's actor made me feel pity for the character yet making sure that we know how evil he is, was done so artfully I had to be impressed. Also ji shuran's actor, she played the tropey stepmother role but still managed to shine though even when wu jinyan was delivering a spectacular scene.
2. The plot, characters, and pacing: Cdramas are getting good at pacing and its nice to see this being reflected in The Double. The plot had significant amount of twists and turns to keep the viewers engaged till the end. The diverse range of characters gave this drama life as well as the character developments. Not everyone received justice or happy endings but you understand the motivations and aspirations behind their actions, even the villains. The double is one of the few cdramas where morally grey characters were portrayed brilliantly. I also liked the subtle emphasis on how systematic misogyny, the effects of bad parenting especially by the fathers towards the daughters, the implications of sexual violence even towards the main character and how the producers did not shy away from it.
3. The OST: Chenle's "Live on love on" was solid art. The opening theme was beautiful as well.
4. The cinematography was better compared to other youku production.

Disliked:
1. Although the plot is pretty solid some times you have to suspend logic to appreciate the drama in my opinion. The slapstick comedy at inappropriate times did not help. Them using wugong for not other reason than being flashy was seriously weird.
2. The leads had excellent chemistry and the romance progression felt natural but did not at the same time which is understandable given romance is not the focus of the drama but I wished there would have been more substance. Like in LLTG although there was an overarching plot the romance was also prioritized as the ml and fl gets entangled early on and it feels more developed. But with this, other than the chemistry and the subtle understanding by the audience that the leads are falling in love, there is not much to go on. It feels like the romance has taken a back seat.
3. The ending felt weird and unnecessary. I know they did that for the dramatics as this is a very soap opera-ish drama but the ending was literally not needed. They could have ended with XH and XF married, a time lapse for 1 year, then cut to them visiting tong'er and jiang li's tomb with a voice over and that would have been enough. That battle scene should have been shown early on which would have increased the angst. The slow burn would have worked out better in that way just like how it did in LLTG. But a scene at the end just goes to show they just wanted to create some hype and nothing else.

It's quite an enjoyable drama and probably the best one in this year judging by the cdrama drought.