I hate my Holga and I will never use it again. I've had some images that screamed for a more pictorial treatment, but the Holga just ruined them with light leaks, unflat film, overlap between frames, etc. I've tried taping the thing up but it still plagues me.
I''d rather buy a decent older camera, violate the lens with vaseline or a vignette mask or scratches, and shoot that.
There are some tricks to fixing the light leak and loose film issues. If you open the Holga and flip it upside down, there are two holes in the light box, where the fake "prism" box is. Put some tape over those. That is a big source of light leaks. To keep the film tight, rip off one of the flaps from your film box, fold it up, and stick it under the film reel after you load the camera. That should tighten the film up. Loose film is probably causing your overlap problem too. I love using my Holgas. It helps when you get stuck in a rut.
Apologies if you already tried these things. Not much quality control with the Holgas, and YMMV.
For double exposures (as I can't do that with my SLR and it seems to be quite cumbersome with my Polaroid as far as I understood) or when I think it'd give an extra dimension to an otherwise too cold image.