Overlapped pictures are caused in the camera when the film is not fully advanced. The last picture on a 35mm roll overlaps the second to the last picture. This is caused by the fact that you have reached the end of the film and should not have taken the last picture. This usually happens when more than the stated number of pictures are taken on a roll, i.e. 26 exposures on a 24 exposure roll. This is also sometimes called a double-exposure.

Source:Ghostresearch