Overwriting objects in the same depth

Lets say I had movieclip a located in depth 400. Now, I attach a new movieclip, movieclip b, on to depth 400 (same depth as movieclip a).

When this happens, I know that a is written over with b. Does anyone know if this type of method actually removes movieclip and will not create any memory leaks or anyhting of the sort? Or does it just overwrites what is on that depth, but movieclip a is still in memory?

I know I can remove movieclip a with removeMovieClip(), but I was just wondering if I really had to in the case above.

What I am really worried about is if this operation is performed many times it may cause some errors or flash may soak up resources.

Thanks for your help and time in advance.