In that case, if you're using an emulator, make sure your save type is set to "auto-detection".
I might add that as far as I know, Pokemon games usually use 512Kbytes (AKA 4Mbits) save format, so you can try to set the target format to Flash 512kb and it might make the detection easier, both on an emulator and a real DS.
Plus, as far as I know .duc is an action replay format, so they probably have some converter floating around.
You may try this (sorry if it's illegal to post other forums' links, just delete it if so):
http://forums.maxconsole.net/showthread.php?t=26252