Did you r-click on the mask in preview window? Or the mask in the layers area?
Whichever you tried, try the other! I'm almost sure I've done it before...
Also, try using keyboard shortcut/menus (ctrl+c/command+c).
Hope this helps!
thanks for these. haven't had a chance to try it yet but the help link is very
You can also make a pickwhip expression to clone the mask shape between other masks/layers.
It's a personal favorite as it keeps the shape of all linked masks identical.