Help: duplicate a movie CLip

hi all, i m new in this forum and i congrats all for the good work.
well i m here to ask a “simple” doubdt i think, i wanna duplica a movieClip and acess it in diferent X positons, and how can i put my mc visible in some frames and invisble in others?
i ve tryed mc.visible = false but it didnt work :S
ty all