That’s the way it works. If the alpha that’s needed for the matte is in the background image you need to apply it twice. The image can’t be used both as a travel matte and a background image simultaneously. If you’re really bothered by the extra layer you could set the alpha channel to the background image in the timeline to none. Apply Image Mask to the video and then then use the background from the browser in the Image Mask well set to alpha. This will only work with a still image.