Duplicate container doesn't work, how come?

Hi there,

Been awhile since i last used flash mx. But these is what i want to do:

1 Put an empty mc on the stage (called container) <- done
2 Dynamicly load a .jpg <- done
2a The .jpg needs to be scaled down 50% <-done
3 Masking the container <- done
4 Duplicate the container, so the jpg can be used as a background <-??
5 Making a transition with a mask <- what i finally want

ad 4 Tried these container.duplicateMovieClip(“containerBG”, 999); but this doesn’t work

Any tips