Mamiya RZ67 shutter release issue

I have two Mamiya RZ67 bodies and one of them has developed a strange problem with the body shutter release - it needs to be pressed TWICE to fire the shutter. If I'm using a cable release in the lens, which I almost always do, then of course the effect is that it takes two presses to raise the mirror, then a third with the cable release to trip the shutter.

Anyone else had this issue, and now how to solve it ?

Thanks for any advice

Kevin