ML Falls for the FL More Than Once

SPOILERS - the ML first encounters the FL in Switzerland. His fiancee (they have an arranged  marriage) is visiting him but he's not interested in her and spends most of their time together looking elsewhere and taking pictures of the scenery. While taking pictures, he's distracted when he sees the FL and takes her picture. And then he hears her recording a suicide message and interrupts her to ask her to take his and his fiancee's picture. He continues to think about her while his fiancee is visiting and from time to time thereafter. He does't recognize the FL from their encounter in Switzerland when he meets her again, but quickly takes steps to protect her (and fully falls for her soon after) again.

The ML and FL were extremely close and precious to one another when they were young children. The ML looked after the FL and protected her. The FL did not speak for three months after they were separated, and looked for him everywhere she went.

They fall for each other when they meet again as adults, even though they do not remember the time they spent together as children.

This one is a bit borderline. The ML and FL were inseparable best friends when they were young children before they were separated by tragic circumstances. They fell for each other when they met again as adults even though they didn't recognize each other.  

The first incarnations of the ML and FL were deities. The ML fell in love with the FL but their love was doomed and ended tragically. The ML fell for the FL again when his soul was placed in the body of the 8th prince and the FL's body travelled back in time to possess the body of the woman he was betrothed to. When the ML and FL enter the spirit world again, the ML loses his memories of the FL. But he is so moved by her devotion that he takes medicine at the risk of his own life in order to restore his memories. 

The wormhole from the end of the first season sends the ML into the FL's past. He meets a version of the FL who has not met him yet. He still loves her and protects her. They are both sent into the past just like the FL was in the first season, but the ML's presence distorts the timeline. An alternate version of his past self also exists in this timeline. That version of the ML also falls for the FL.

The leads didn't know each others' names when they were children and through a misunderstanding the ML thought the FL's friend's name was the FL's name so they don't recognize each other when they meet again as adults. Due to certain circumstances, the ML is advised to try and look for the differences between the FL and the girl he knew when he was younger, and it takes a long time for the misunderstanding to be resolved. He falls for her again during that time.

The ML and FL meet as children and fall for each other. The FL is later forced to pretend to be a boy and take her brother's place as crown prince. When she meets the ML again, she tells him that the girl he had met before died. The ML fell for the FL again while she was pretending to be a man / held the position of crown prince without knowing that she was actually a girl (particularly his first love). 

As a child the ML sees the FL as his saviour - someone who is beautiful, just, precious, and someone he must protect.

When he meets her again he is in high school and still has a crush on his idealized version of her in his memory. As he gets to know the real her, even with all her flaws, his feelings remain and deepen.

The ML is in an accident and suffers from amnesia. He is accused of murder and the FL zealously defends him. Even though he can't remember her, he falls for her again. 

In the original timeline, the ML falls first followed quickly by the FL, but their romance ends in tragedy and the FL is forced to reset the timeline. Some things are slightly different in the new timeline and everyone other than the FL cannot remember the original timeline, including the ML. Although he is suspicious of her at first, the ML quickly falls for the FL again.

Doesn't know she's his childhood crush

Childhood first love / amnesia 

Multiple lives / identities + FL's amnesia

Multiple lives

The ML says that he fell for the FL several times and gives examples, although I personally didn't find them very compelling.