Recommendations

Both dramas involves two non-relatives twin women (rich woman and other is poor from each drama), of which the rich woman from each drama engaged to the son form rich family but for business purpose and not real love but later on due to different reasons the poor one forced to be married to those chaebol sons and replaced the rich ones. In bride of century JYK didn't want to be married by CKJ so NDR replaced her. In mask, due to the coma happened to SEH and killed by her in-law, she was replaced by BJS. Both those sons from each drama had no feelings to these poor women in first place but falling for them later.

both are about a forbidden love story between a man and a married woman.. both have r rated scenes

it have almost the same names. It most the same exact same thing.Also both have powers.

In both dramas a baby is left with multiple men who could possibly be the biological father and take care of the child together.

It has a Veryx100 similar plot and any person who has watched 'the promise' would straight away realise the similarities between the two dramas.
1.Both leads lose a parent
2.both also have a biological parent without knowing
3.get betrayed by boyfriend who chooses another women with the intention of becoming successful
4. Lose a child
5.revenge begins ,to take down the whole family and take over the business.

Both dramas feature time travel to a different era.
In Tunnel, granted it is just from 1986 to present.(still a big culture shock)
In Faith it is to the Goryeo era from the present.
Both bring a little star power with their Brave and Manly leading men.
Faith's cast is lead by Lee Min Ho playing a General of the army and bodyguard to the King.
Tunnel's cast is lead by Choi Jin Hyuk playing a tough Detective who is not afraid of tackling a dangerous criminal. He is also a respected sunbae for his less experienced partners.
Both have a lead who is faithful while longing for their loved one after being separated by time.
Both are directed by Sin Yong-hwi
Both are really good dramas worth watching.

Where they are different:
Tunnel has a serial killer crime story and slightly dated outfits.
Faith has political intrigue, medicine, and of course the long hair and historical dress.

Another gumiho story with the star lead as lee sung gi.. Really good...

Very similar to heartstrings.. Revolves around a set of students where the central theme is achieving their dreams and music.. And most similarity is the music itself.. Both have wonderful music.. Very calm and warm.. Worth a try..

Both series have similar themes of spirituality as a source of power. Maskman tends to be regarded as the prototype for Dairanger due to it's similarities in regards to themes and story beats.

Both are crime dramas produced by tVN, and both are inspired by the movie, "Memories of Murder".

They Both are both shows that have some medical scenario intertwined in the story. Both are very complex rom-com's on top of that. You'll fall in love with Jo In Sung as the male lead just as much as you fall in love with either of the leads in Jealousy Incarnate. It just happens to be that Gong Hyo Jin is the female lead and is perfect in both. If you haven't watched this, drop everything and watch it.

- both the main characters are good at what they do
- both the main characters loved someone they grew up with
- both main characters lives were screwed with by people they knew
- both main charaters were seperated from the ones they loved
- both main characters fathers are dead

Despite the name "Five Fingers" its actually really good. It's also a drama about revenge. It also has the same lead. The main lead plays a sort of instrument in both dramas.

Personally, I really like Nine: Nine times Time Travel :D and after watching first episode of Tunnel, I am already addicted to it!~ Those who love time slip dramas will definitely love this too :) (if you aren't too picky) It brings me laughter (natural type of comedy), tears, nervousness etc. while being intense. While Nine:Nine times travel time travel via incenses, Tunnel is via Tunnel as the title mentioned.
Cases were interesting.

Both have Kubozuka Yosuke
Both have this kinda Japanese 90s filming atmosphere
They sometimes give the same feeling, although IwGp is a drama and Go a movie