How to make this effect?

[hello0]

The effect where that girl melts into herself with a still image of herself

[JVRaines]

Use tracks with alpha transparency. From the bottom up: background, moving figure, stills. The stills are animated to coincide with the background and to meet the moving figure at the proper frame.

[hello0]

The stills are like transparent layed over the video and when the moving figure meets the still, the still disappears at the right time and the moving figure moves on?

[poisondeathray]

The stills are like transparent layed over the video and when the moving figure meets the still, the still disappears at the right time and the moving figure moves on?

Yes, the still layer is edited (cut off) at the point where it meets up with the live action footage. Each insert can be edited (the in point and out point). Each still image insert is masked out (rotoscoped) from the original footage. The kpop video example did a bit of color manipulation to enhance the transition

But you need to 3d motion track (camera track) in order to get the still to "stick" with the camera motion, as if it was really there. If there is camera motion, it's very difficult to hand animate this effect and have it look good

This effect is known as a "floating freeze frame". It was made popular by Clubfeet and more mainstream by One Direction music video.
clubfeet: https://www.youtube.com/watch?v=SPffFTWZWlI
one direction: https://www.youtube.com/watch?v=_kqQDCxRCzM

There are many tutorials on how to do this step by step. Usually it's done in after effects
eg.
https://www.youtube.com/watch?v=hD5mxH0a1L0