Technically you are not guaranteed to get the same backbuffer preserved after you do a Flip(). The o/s might allocate several back buffers and cycle through them, or you might not even get one. Assuming that its contents will be preserved is nice and efficient but I would expect that on some systems the screen will flicker and swap between out of date images, or show old unwated graphics.??
I don't see why it should happen. You only have one dedicated buffer and the OS does not write wherever he wants. Double buffering has been used for more than a decade... setbuffer backbuffer() would be useless otherwise.
Simply put, the alpha rectangle fades the last frame, then the next is drawn without clearing the last one. When this cycle repeats, the alpha rectangule simply fades the current one plus the older frames. However, old frames are overimposed in the same buffer. This method in only practical with blank backgrounds, but works nonetheless.