I just wrote a script that does that called "Duplicate and move layer." Download the script from this link and place in your Presets\Scripts folder of Photoshop. Restart Photoshop, and it will show up in the main menu under Files>Scripts. You can make an action to play it then create a hot key to make it easier. What you do is select two layers. A dialog box will come up and you select a button to select which of the selected layers will be duplicated and moved to the other selected layer.
This link may only be good for a short time, as I may post this to the new Adobe Add-on site.