sorry for resurrecting this thread, but i was looking for the same when i discovered it and would like to document my solution for the original problem that i was planing to solve with the duplication of the way: the tedious creation of duplicate ways for use in areas (where it makes sense). to do that one has often to create a way with some of the exact same nodes as an existing way and some additional nodes. think of a forest with a complex border that is mapped already and you want to map the meadow beside it. thanks to IRC i discovered http://josm.openstreetmap.de/wiki/Help/Action/FollowLine which is very well suited for this kind of problem.