I only finished the last 3 episodes at 2X speed mainly because I was too in love with Yang Se Jong's acting in this drama. That, and the two main clone characters were the only reason I continued to watch a drama which rapidly deteriorated towards the second half, after a promising beginning. I don't know how he did it, but Yang Se Jong beautifully managed to play three roles with their distinct personalities and it was so majestic to watch. You could immediately tell which one was Sung Joon, which was Sung Hoon and Dr. Lee (and it's not because of the different hairstyles.) I could write so much on his acting skills here but I'll keep it short.
Lee Sung Hoon's pain, anger, disappointment, fear and his loneliness were all palpable and moving. I felt for him right from the beginning, even when he was presented as a villain, because I could tell there was something more behind his anger and malice.
Let me highlight my main problems with this drama:

BLAND CHARACTERS:
The other characters aside from the clones were unfortunately too bland, annoying or unlikable. I could not stand Prosecutor Choi's creepy smile (which was the only expression she had by the way) and ended up speeding past her scenes. I couldn't feel anything for Detective Jang or Soo Yeon. The actually villains were just greedy, selfish people without any other side to them. It was boring to watch other characters except the clones.
GLARING PLOT HOLES:
I'm always after a well-developed plot so this was very painful. There were too many questions I was left with. The sheer amount of times a character got hit by a speeding vehicle was ridiculous!
Why was there a need to create two clones of Dr. Lee?
Why does one clone have all of Dr. Lee’s memories but not the other?
What exactly did Sung Hoon do to Sung Joon that he lost all of his memories? Why doesn't the latter even care about it?
What the heck is wrong with prosecutor Choi? She is heartless and imo the real villain in the drama.
Why didn’t the researchers try to retrieve Dr. Lee's organs right after they were stolen?
Why are human organs treated like candy you can just steal from one another?
Why wasn’t there better security at the lab (back in 92) considering it was illegal experimentation? How were they able to casually take out all the organs of a top doctor and walk out with them? No, but how is it that Dr. Lee’s organs did not reject the bodies of the new recipients?
The "vaccine" seems to be some all-purpose miracle cure that can cure all kinds of cancer, tumors, ageing organs and so forth.
Characters keep fake dying and returning back to life.
What kind of security team (at the research lab) doesn’t have guns or any other weapon?
Dr. Han Yu Ra always being overseas during critical times e.g. daughter’s death and husband’s death. She’s pretty clueless about a lot of things.
Mi Rae stopped being curious about her mother at one point and doesn’t seem much bothered that her mother was involved in illegal experiments on convicts or kept such huge secrets from her.
Mi Rae is a reporter but has also studied medicine, and doesn't look like she's playing the part of either? I mean, are reporters usually this naive, extremely trusting and kind?
The chairman is selfish and greedy, yet his surprised that his daughter is selfish too? That was messed up.
Detective Deuk Cheon got shot in the gut but is already up and walking in the next episode.
The last few episodes were so rushed that I feel angry Sung Hoon didn't get more time to work out his feelings. Dang.
Sung Joon is totally fine in the last episode and it's not even clear whether he got the bone marrow vaccine or not? At least insert a dialogue to make it clear.

In short, don't watch it if you're looking for an action thriller with a good plot. The drama had little to no cliffhangers in the second half and I had to drag myself to click the next episode.