Hmmm, based on my own incident with a FED and like RXMD suggested, have a look at the sprocket holes around where the exposed frames stop. If there is damage and/or scratches there, then it suggests the un-rolling of the film was being impeded by something, hence making the sprocket wheels skip. In turn, that would allow the shutter to reset without moving the film the full distance, and so give overlapping frames.
If a very stiff rewind shaft was the cause, then Payasam's fix would do the job. There could probably be several other causes of course, dunno what exactly though as it was a similar problem on the FED.
If there is no damage at all on the film, then it suggests that there is a hitch at the 'other end', with the wind-on gearing/clutch etc.
Errrm, that never happened to me but I am guessing on it as a possible cause . . . I suppose it could be possible for the shutter to be wound while the film-sprockets slip - maybe the clutch that drops out for rewinding the film is skipping ? (But wouldnt that also stop the shutter winding?).
Good luck !
😉
