Jason beat me to answering your question. As he said, you did the correct thing using Save As at first (since you do not have permission to modify problems in the NPL), but once you own a problem you can edit it and Save the new version or Save As under a new file name. But if you try to Save As to an existing file, you get the error you saw.
I have moved your email and Jason's reply to the forum (http://webwork.maa.org/moodle/mod/forum/view.php?id=4) so this information is available to other people. The forum is a good place to post such questions.