[senocular]

ActionScript 3 no longer has a duplicateMovieClip method for MovieClip instances (or any DisplayObject instances). Instead, it's suggested that you just create a new instance of the display object you wish to duplicate using its constructor. This, however, is not the same as duplicateMovieClip, and, really, is more like using AS1 and AS2's attachMovieClip. For a more accurate representation of duplicateMovieClip in AS3, consider the following function:

package com.senocular.display {
	
	import flash.display.DisplayObject;
	import flash.geom.Rectangle;
	
	/**
	 * duplicateDisplayObject
	 * creates a duplicate of the DisplayObject passed.
	 * similar to duplicateMovieClip in AVM1
	 * @param target the display object to duplicate
	 * @param autoAdd if true, adds the duplicate to the display list
	 * in which target was located
	 * @return a duplicate instance of target
	 */
	public function duplicateDisplayObject(target:DisplayObject, autoAdd:Boolean = false):DisplayObject {
		// create duplicate
		var targetClass:Class = Object(target).constructor;
		var duplicate:DisplayObject = new targetClass();
		
		// duplicate properties
		duplicate.transform = target.transform;
		duplicate.filters = target.filters;
		duplicate.cacheAsBitmap = target.cacheAsBitmap;
		duplicate.opaqueBackground = target.opaqueBackground;
		if (target.scale9Grid) {
			var rect:Rectangle = target.scale9Grid;
			// WAS Flash 9 bug where returned scale9Grid is 20x larger than assigned
			// rect.x /= 20, rect.y /= 20, rect.width /= 20, rect.height /= 20;
			duplicate.scale9Grid = rect;
		}
		
		// add to target parent's display list
		// if autoAdd was provided as true
		if (autoAdd && target.parent) {
			target.parent.addChild(duplicate);
		}
		return duplicate;
	}
}

As you can see, this function (duplicateDisplayObject) takes care of making sure a duplicated instance also retains all the information retained by duplicateMovieClip such as transformation, filters, chaching as bitmap, etc.

Note: There is currently a bug in Flash Player 9 that causes incorrect values to be returned from the scale9Grid property of display objects. This function compensates for that but may need to be edited should this bug be fixed.

Usage:

import com.senocular.display.duplicateDisplayObject;

// create duplicate and assign to newInstance variable
// using true for autoAdd automatically adds the newInstance
// into the display list where myOldSprite is located
var newInstance:Sprite = duplicateDisplayObject(myOldSprite, true);
newInstance.x += 100; // shift to see duplicate

The only thing duplicateMovieClip does that this does not is copy dynamic drawing information. Currently, the graphics object in display objects cannot be duplicated so there is no way to obtain that information for duplicates in duplicateDisplayObject.