Remove duplicate road

By mistake, I created a road that already existed (actually, the sever was so slow that I thought that the road was missing).
Can someone explain me how can I remove the duplicated road?

Which editor?
JOSM: Download area, highlight way, hit Del, upload changes.
Potlatch: Open area, highlight way, hit Shift-Del, wait for the API to update the changes.
Merkator: No idea.