With ctrl+C/ctrl+V, you have to press two key combinations. Ctrl+shift+D does this in one pass. But basically, they do the same thing -- you have a copy of the object sitting right on top of the original!
Alt+drag also copies the object. Difference is that you are moving the object somewhere; on the other hand, pressing ctrl+D, duplicates (clones) the object right on top of its original (exact same place).
It's a matter of personal preference and what you intened to do with the copy of the object; but basically, all three options you mention, do one and same thing: they make a copy of your selected object. Of course, pressing ctrl+C and then ctrl+V will work if you intend to copy the object and then paste it in another place/another PNG document; in this case, using ctrl+shift+D won't be a good option, as you'll have to first make a copy of the obect (ctrl+shift+D) and then again, cut or copy so that you can paste it (ctrl+V) in the other document...
Maybe some other Fireworks guru can shed some more light on this question, but this is how I see things...
Thank you for your answer.
Alt+drag i can understand, but ctrl+c/ctrl+v and ctrl+shift+d seen to be exactly the same. As long i could see, both copy attributes and proprieties from the source. So, I can't see any difference.
Anyhow, Thank you very much.
Ctrl+c/ctrl+v can be used to copy an object to another file or layer, etc. Obviously, ctrl+shift+D cannot be used in these cases, as the copied object will be left as it is...