Overlapped frames with GSN

[javimm]

Hi. I'm having a problem with my GSN. I've shot 5 rolls with it, and in 3 of them, there are two frames that are overlapped. I mean, no space between them and 25% of a frame is over the 25% of the next frame. It happens more or less at 18-23th frame of every roll, for no apparent reason, and it only happens once in a roll, just in two consecutive frames.
I don't do any special thing, just shoot, wind, shoot... No hitting anything in between.

What can be the cause of this problem?

Thanks.

EDITED TO ADD: The light seals are new. I've changed them, and there are no light leaks. The frames overlap in horizontal.

 

[Steve Bellayr]

I have had this happen on different cameras. I attribute it to "operator error" and move on with my life. Years ago, before the advent of digita, photographers were taught to bracket their shots not only to guarantee for proper exposure but also for such errors. Try to make sure that you are advancing the film fully. As the film builds up it is harder to advance. I look for forward to other responses as I have had this problem too.