Hi,
I believe that can happen if a device is scheduled for removal, but hasn't actually been removed from the database yet.
In memory, it will be gone (hence you can't see it in the Console), but it is still in the database (causing conflicts when you try to re-add the node).
The syncer thread which runs every minute by default should write updates to the database, but I guess it could be running into problems.
Try setting debug for obj.sync and sync to 7 and watch the NetXMS server logs after you delete the object.
To enable debug open the NetXMS Server Console and enter "debug obj.sync 7" and "debug sync 7".
An example of a deletion gone wrong from our logs:
Object 120.150.216.0/24 [205198] marked for deletion
Unable to delete object with id 205198 because it is being referenced 8 time(s)
In our case the above was removed after about an hour (we run the syncer every 5 minutes), presumably because the references were eventually removed.
Cheers
