1 movie clip superimposes another, I don't want this

Does anyone know how to prevent one m/c from superimposing on another movie clip?

I’ve got a thumbnails m/c(when clicked to show a bigger pic of that thumbnail in the middle of the screne) that superimposes on another thumbnail m/c.

Both m/c’s have larger pics of their respected thumbnails, but when one is clicked from one m/c and a thumbnail is clicked from the other m/c their pics are both on screen.

How can I stop this?

Thank You.