duplicateMovieClip problem

i have movieclip chats_mc added to chatting_mc.i used following code to duplicate on click event of a button(nos increments each time).

chatting_mc.duplicateMovieClip("chats_mc","chats_mc"+nos,chatting_mc.getNextHighestDepth());

when im tracing like this im getting “undefined”

trace("NUM>>>"+chatting_mc.eval("chats_mc"+nos));

i want output sumthing like

chatting_mc.chats_mc1
chatting_mc.chats_mc2
..

Can anybody point where im making mistake…or give an alternative to this…