Double-check the rules for the list. In my Outlook 2000 I do have the option to check both "move an entry" and also "move a copy of an entry".
I'd never noticed that before ! Nope, it wasn't selected.
If you're on XP: Start/Control Panel/Mail. Click on the Addressing tab. Make sure these settings are what you want.
That gets me to the same options in Outlook that I've already been through to no avail :-) Can't believe it would let something get setup like this with no way to correct it - oh wait, it's MS :-) Thanks ! Chuck