Thanks everybody for your replies. I'm quite new to this camera so I don't have statistics on when and where it exactly happens: I went through four rolls only, and only two of them showed overlapped frames. The overlapping happened toward the middle of the film in both cases. In one case, the film was slipping over the sprocket continuously so I couldn't wind it to the end and had to quit photographing.

It may perhaps be that I didn't pay enough attention on how well the film was secured to the takeup spool. Perhaps the lead-in slipped and the loss of traction from the takeup spool may have been sufficient for the advance mechanism to stop operating. Just guessing. I will go through another roll soon using maximum care during loading and will see what happens.