Multiple user avatars
Assuming that you want to use the same avatar image, just in different sizes, you do not have to duplicate anything.

Being able to display the same image with different styles is the whole point of having different image styles. "Styled" images are created on demand, according to the request. This already works out of the box for normal image files (For example, you can chose a different image style for teaser and default view modes).

The user avatar image can be displayed with different image styles too, the only problem right now is that you can't select the image style when using theme_user_picture(). There is however already a patch for that. And if you're using Views, it should already be possible to choose the image style for the user picture.