it depends on when u want it done
for example:
if you want it done when you click some where

this.createEmptyMovieClip("canvas_mc",1);
var mouseLisener:Object = new Object();
Mouse.addListener(mouseLisener);
mouseLisener.onMouseDown = function(){

canvas_mc.attachMovie("paulse","paulse_mc",canvas_mc.getNextHighestDepth(),{_x:
_xmouse,_y:_ymouse});
}

edos12, that code will attach a movie from the library on mouseclick and place
it at the location of the mouseclick.

for duplicating a movie clip that is already on stage and set its x coordinate
to 53.8 DJSickNick, you can use:

MCtoDuplicate.duplicateMovieClip("newMovieClip",1,{_x:53.8});

where MCtoDuplicate is the movie clip to duplicate, newMovieClip is the
instance name of the new movie clip, and 1 is the depth you want the new movie
clip placed.

for more assistance with this, have a look at the
http://livedocs.macromedia.com/flash/8/main/00002452.html