Hi

are there any dramas where the female lead falls for a guy, whos already taken? 

He doesnt have to be an arrogant bad boy, its okay, if hes nice to her too. But he should already have a girlfriend while the female lead tries to deal with her feelings toward him

thanks in advance! :) 

Bride of the Century - She pretends to be his fiance
Fated to Love you- He has a girlfriend that he mistakes the lead for (I haven't seen the Korean version but I assume it's the same set of circumstances)
Drunken to love you- He has a girlfriend he proposes to, she says no and he meets the lead.
Woody Sambo- He has an on and off again girlfriend when the leads meet, I think they are broken up but he's still trying to be with her.
I Really Really Like You - It's not the main lead guy but her feelings for him will play a big part throughout the drama. He has a wife and a kid but since the wife has dementia, things get complicated.
Witch Yoo Hee - But he falls for her, too.
Flower Grandpa Investigation Unit - She's a supporting role but still the most important female role in this drama -  and the romance is centered around her. She likes someone who's about to get married.
Ruler of Your Own World

Such a great drama, but so underrated.
Thank you everyone! :) Ill take a look on your recoms.