Title says it all

Same thing for me, I've never ben able to duplicate a worldspace in FNV. It's a drag because I can't build my "Glass Windows" Cells because of this bug.

I don't think the GECK will ever allow you to Duplicate any worldspace, you'll have to make your own from scratch.

Sorry

cev

I went through this little excercise and it seems to be fine:

To copy Boulder City Worldspace
open FalloutNV.esm with FNVEdit
Drill into worldspace, highlight BoulderCityWorld
right-click, deep copy as override, into new file. For a large worldspace, you may have to do it in chunks till you get it all.
Close and save.
Reopen new plugin with FNVEdit
Drill into worldspace, highlight BoulderCityWorld in your new plugin.
Press SHIFT * to expand the entire tree.
Under the very first cell, highlight the very first reference under 'persistent'
Scroll down to to bottom
Hold the SHIFT key and click on the very last cell. all the cells and persistent references should be highlighted now.
Right click on a selected cell - change formID
Select your new plugin as the target.
All the prefixes of the cells will change to 01, and they will no longer be overrides to the main esm.
Go back near the top, and highlight the very first reference under 'temporary' that you see.
Go all the way to the bottom, hold the SHIFT key and select the very last temporary reference. If this is a large worldspace, you may have to do this part in chunks, or you will run out of memory. Do a chunk, save, exit, reload, and do some more. For BoulderCityWorld, I can do it in one selection.
Right-click on a selected reference, 'change FormID'. Select the new plugin as the target
When this is done, you should have only two override records left:
0014C723 - BoulderCityWorld
0014C74D - the persistent worldspace cell
Change their form ID's individually, but before doing that, do a search to make sure you won't get a duplicate form ID. In the top left, search for 0114C723 and then 0114C74D
In this case both of the new form IDs I want to use aren't taken, so right click on the 0014C74D persistent cell, change form Id, change the first two digits to 01
Then right click on the BoulderCityWorld record 0014C723, change the form ID to 0114C723
Highlight BoulderCityWorld and on the right, change the editor ID of BoulderCityWorld to MYBoulderCityWorld or whatever so that is different than Vanilla
Close and save.
Reopen with FNVEdit
Make a note or screen shot of all the persistent references. DON'T delete them here, unless you are confident you can clean up the errors that will result. close FNVEdit
Reopen with the GECK. Delete all the named references from your copied world - example BCPrivateGilbertRef is a named reference. You will see it as BCPrivateGilbertRefDUPLICATE000. Mainly scripted objects that could affect quests.
Remove any door teleport destinations from all the doors. for Boulder, there were two - one to the outside world, and one to the Kahn hideout.
Fixing the doors this way creates dirty edits on the vanilla destination doors. Open with FNVEdit and remove them.
Once you fix the doors, edit the navmesh in the Geck - pick a vertex in each cell and finalize it so the green lines appear.
Save and the Geck should create a new NAVI record for the navmesh - you can check this with FNVEdit.