I don't think you can achieve that using Actions, your best bet is creating a Script for that. You can use purely Script for this, but if you still want to incorporate Actions into it (as the trigger), you can set the Action to use a Script after duplicating the current layer.
Open up a text editor, give it any name you want (the Script's name is the same as the filename), and paste this in:
app.activeDocument.activeLayer.opacity = app.activeDocument.activeLayer.opacity / 2;
Save it as a
.jsx in your Scripts' folder (
X:\~\Adobe\Adobe Photoshop XX\Presets\Scripts), and you can select
Insert Menu Item from the Action menu to insert the script after the duplicate action.