(Will contain spoilers)
Similar:
-Major Character Death's
-Crime Dramas
-Cops in both
-One character walks the line between being good & bad
-Child Abuse, adoption, selling organs/children

Not Similar:
-Duel involves clones
-While Duel has clones one is good, however the other is bad (but can be seen as good) whereas Kill It's main male lead is the one who is bad (but can be seen as good)
-Female Lead is the cop in Kill It, Male Lead is the cop in Duel
-Duel has a more fantasy/science-fiction feel to it whereas Kill It is more realistic

Simply because the robot is basically a clone of the main character.
Like in Duel Sung Hoon and Sung Joon are clones.

Both are supernatural.
In While You Were Sleeping the female lead sometimes sees people dying in her dreams.
In About Time the female lead sees the life clocks of everyone around her.

Both are about seeing death in way.
Black the woman sees a black shadow.
About Time the woman sees the life clocks of people.

Both feature robots. Instead of a a male robot it is a female one. Both robots are molded after real people.
Both dramas are romance.

Band of people get together to take down criminals. Both have criminals.
In Lookout (The Guardians) the main female lead is a cop but is a wanted criminal. In Bad Guys there is a group of criminals.

A crew of people band together to take down bad guys.
In Mad Dog the main focus is insurance fraud and in Bad Guys their is a group of criminals going after other criminals.

It's season 1 of Bad Guys so it's definitely bound to be extremely similar. Different cast. Same plot line, bad guys taking down other bad guys.

Has one of the same actors from Healer. Similar vibe to Healer. No masked characters though. Instead of one person there's a group of people.

Yeo Jin Goo plays the main male lead in both.
Both have a mystery and supernatural elements.
Similiar in a sense that both drama's have two parts in them.
Circle has part 1 Beta Project set in 2017 and part 2 Grand New World set in 2037.
Reunited Words has parts in the past in 2005 and parts in 2017, however not in a set sequence like Circle.

Both dramas involve a person "traveling through time" in a way.
I'm not sure if Reunited Worlds has anything to do with time-traveling yet but it could.

Both dramas involve a person "traveling through time" in a way.
In Hit the Top the male lead travels from the past into the future and doesn't know why. He doesn't realize he's in 2017 at first but he does after a while.
In Reunited Worlds the male lead wakes up in 2017 thinking he's still in 2005 where he was still a high school student. He doesn't realize this at first.
Hit the Top is different since it's a time traveling drama.
Reunited Worlds seems more supernatural.