Who downloaded the data first is completely irrelevant. Depending on how much time "N" has he can either: - simply revert C2 - check all objects in C2 one by one to see if they are somehow better than the existing objects (including those from C1) and remove them if not. If you are using JOSM (looks like it as this workflow is (nearly?) impossible with Potlatch), then you should [update the data][1] from the server just before you upload, if you have taken a long time to make your edits. [1]: http://josm.openstreetmap.de/wiki/Help/Action/UpdateData

Who downloaded the data first is completely irrelevant. Depending on how much time "N" has he can either: - simply revert C2 - check all objects in C2 one by one to see if they are somehow better than the existing objects (including those from C1) and remove them if not. If you are using JOSM (looks like it as this workflow is impossible with Potlatch), then you should [update the data][1] from the server just before you upload, if you have taken a long time to make your edits. [1]: http://josm.openstreetmap.de/wiki/Help/Action/UpdateData