With the current Cocoa Tk 8.5 (ActiveState 8.5.15), it appears that the IDLE Redo accelerator, Cmd-Shift-Z, has the same effect as the Undo accelerator, Cmd-Z.  This is probably related to the behavior and changes in Cocoa Tk noted in Issue11055.  With the older Apple-supplied Tk 8.5.9 on OS X 10.9, the Cmd-Shift-Z causes two Redos.  Cmd-Shift-Z works correctly with Carbon Tk 8.4.20 (as linked with the 32-bit-only python.org installers) and selecting the Redo menu item, rather than using a keyboard shortcut, appears to work correctly with all versions.