Reply To: "Duplicate" keyboard command stopped working

Shortcuts work differently in different sections of a document. go into the Edit menu and click Keyboard Shortcuts. From here you can find duplicate in the Product Area:Edit Menu. Check that the Default: shows Shift+Cmd+Opt+D. If not you can create the new shortcut in a Default Context:.

I've had somewhat of an issue with this myself, though instead of shortcuts failing to do what they are suppose to do they had somehow been assigned to something else.