i think they meant underexposed when they said spoilt. so unless you have a flash, try to take photos outdoors in bright sunlight.
yeah a photo can have 2 or more images. it's called multiple exposure. not all cameras have that function. dianas and holgas can take those kind of photos. you just need to take 1 shot, then instead of winding the film to the next frame, you take another shot so that both shots overlap.