I have had the same thing happen many times, just use the alignment tool,
very handy for this kind of thing c
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When i dulpicate an object across horizontally or vertically (holding down
Option, Shift), the duplicated object usually doesn't stay aligned with it's
original, see

http://home.exetel.com.au/twistedpancreas/images/color_change.swf

this has happened also in previous versions of Flash (I'm on Flash 8 currently
and on a Mac, but it happens on PC's too)

but i've never had the opportunity to ask why this is.

So i will now.

Why is it so?

I tried to replicate this issue unsuccessfully. It worked for me exactly as it
should. Flash 8 pro on XP. I even checked the info box coordinates and they
checked out. Here is a screenshot:

http://www.johnkalnin.com/flashhelp/flashdup.gif

You can even see the dotted line showing it is in alignment.
Do any of you have any more details on how to get this to occur?

I also get the alignment offset when duplicating and using shift to constrain
the movement. No solution, though I wish I had one. It's seriously slowing me
down. I've tried looking to see if I have anything set to snapping, but that's
not it. Very rarely it will behave as it's supposed to, but that's the
exception.

Yeah i did a little test, and it seems to happen more when you zoom into an
area.

i also found if you try to select the centre of the object and constrain it,
it usually doesn't jump somewhere else. I think converting it to a symbol also
helps so that you know where the centre point is.

But yeah if you select an edge of an object and try to constrain it seems to
jump somewhere else, which can be a bit frustrating. Sometimes it just means
zooming out (like 800%) and holding down Shift and using the arrow keys to
constrain the movement.

Hope that helps a little.