Add a "Duplicate File" command

[c0mmander8]

Add a "Duplicate" command with an Apple-D keypress shortcut to the File menu.

[c0mmander8]

Also, dragging item(s) within the same window while holding the Apple key down should make a duplicate of it or them.

[inexorabletash]

Good First Bug notes:

Start off by repurposing the logic used for Rename and New Folder, adding another variant to prompt for the name. Then invoke the copy logic which is already set up to be called with a source and destination path.

[ph0enix-c0der]

"Also, dragging item(s) within the same window while holding the Apple key down should make a duplicate of it or them." is now issue #837.