Duplicating forest background and foreground

In general, if this is just a still shot I would never recommend using After Effects for this type of work. While AE does have the ability to duplicate and mask layers and even has a clone stamp tool to clone areas of a layer to other areas it isn't really intended for this type of work on still images.

A better choice would be to use photoshop for this type of image editing. It has all the tools that AE does but many other specialized ones like generative fill, content aware fill, spot healing and the patch tool that would make this type of work much easier and quicker.

I would prefer using adobe photshop Generative Ai to fill the area.

expand the canvas, make a selecfion , press generate .
Give it a try, the reult will be outstanding!

Just duplicating the left trees will inevitably show recognizable repeating patterns and you will need additional footage/ photos. Otherwise it seems perfectly doable, assuming the footage is high resolution enough and not just the tiny image you posted. It's just not going to be a one-click thing.

Mylenium