How to Duplicate Layer in After Effects

You can duplicate a layer by selecting it and pressing Ctrl+D (⌘+D).
You can do the same by selecting a layer and pressing Edit – Duplicate.

How to duplicate multiple layers?

You can duplicate multiple layers at once. Make sure to select multiple layers holding Ctrl or Shift (⌘ or Shift) and pressing Ctrl+D (⌘+D).

In case you need to duplicate a composition, check out this post.