Duplicate with Connections (demo video below), which lets you:
"make a copy of a layer and link all its properties to the original using expressions. This is useful for instances in which you need to duplicate an animated layer but you do not want to spend all your time copying and pasting keyframes just to update it. For example, you may want to have a camera in a nested composition duplicating the movement and properties of a camera in a main composition. After running this script you may choose to disable whichever expressions you will not need. Note that expressions can slow down a project, so you should use a combination of disabling expressions and converting expressions into keyframes when appropriate."
There's a couple of related scripts at AE Scripts, DuplicateLayers-n-UpdateExpressions by Lloyd Alvarez and True Comp Duplicator by Brennan Chapman. See also Bake Parent Transform AE, another script by Michael Gochoco, at his website Blurry Pixel [also on AE Scripts, as is his Recursive Scale Comp].