So I imported a movie clip as an image plane and in the shading editor set the clip to start playing at frame 50. Later in my project I imported the same video clip again to appear at a later point in the animation, but when I set the frame for it to start as 150 it also changes the previous version of that clip’s starting point to frame 150.
you can isolate copies by clicking on number of users button:
So you have two objects with same material, right?
In this case you can create new copy of that material and edit it separately:
If it will not help, you can try to share your .blend file (without movie clips but with objects what you try to key)
That worked, Thanks!