The image is loaded twice. If you use a 4-channel image file with RGB for diffuse and the alpha channel for opacity, there's no way to save memory by using a separate Image Map Shader plugged into the 'function' input, because you'd still need two Image Map Shaders so that one can be set to "use alpha channel".
However, if you make your diffuse map a 3-channel image and then use a single-channel 8-bit image for your opacity, you can save memory. You might as well do this with the Default Shader though.
Matt