I have never had any winding problems with my M7... John, do your negatives have tears in the sprocket holes? If so, then it is a tensioning problem, but if not, you might do a little experiment where you open the camera back, apply slight pressure to the film wheels, and wind on. If there is a slight slippage in the mechanism it might be possible to cause the slippage by applying pressure with your fingers. I have an older Exakta camera that has this problem, so I have to make sure to shoot smaller rolls (12 or 20 exposure) only. Actually, I often get some cool shots with this random double - triple exposure 'feature' - but I'd not be so happy if my Leica did it.
Oh, you should also check to be sure the rewind-release lever is fully returned to the upright position... but I presume you already checked this.
Cheers!
- marc