...HELP ! Hexar rewinds on its own ...

[awilder]

Try my suggestion anyway, it couldn't hurt as it seems to involve the mid-roll rewind switch in some way or another.

 

[FrankS]

There could be some film bits in and or around the rotating cylinders or their bearings, causing undue drag on the film advance system. Try blowing them clear with compressed air, but be careful of the shutter - set it on B to get it out of the way while doing this.