My Nikon FM (which I hardly use, but have as an occasional backup) has recently developed an issue: after releasing the shutter, when I wind it automatically shoots the shutter again. Obviously I have to wind again, and sometimes it even shoots for a second time. The result is that most of the times I get two exposures when I just wanted one (the second one being completely useless, of course, since I am normally pointing the camera nowhere), or even three. Sometimes, though, it just works fine. Any idea of what could be wrong here and how to (easily) fix it?