Yes, but there's the relation to the "do-again". Can you summarize each (I think I know ctrl and shift).
Alt is new in 0.40. See Select Under and Drag Selected in http://inkscape.org/cgi-bin/wiki.pl?ReleaseNotes
[...] command". For example one can easily rotate by 45 degrees with copy, then request repeat-last-command 6 more times to get the objects spread around in all 8th of the circle... Making a horizontal sequence of strokes is another example.
That one is indeed very useful. I thought about that. Please search our list of RFEs and if you don't find it, submit a new one.
Careful, it makes only sense if you could actually previously duplicate!
If we add a "duplicate" checkbox to Transform dialog then this will be possible by repeatedly pressing the Apply button there, no need for a general "repeat last action" command (which I generally don't like because it will often give you surprises).