Acivate Secondary click in the settings (click on right side). Then use right click to enter numbers. It stop duplicating digits.
I have the same problem and attribute it bad programming. I have installed other calc widgets and had the same problem with duplicate numeral entires when using the mouse.
It seems like upon releasing the mouse, the calculator reads another click. If you press down on the mouse and drag off of the calculator to release it does not duplicate.
Apple just looks dumb when programming bugs like this happen and go unaddressed. Even the track pad on my MBpro does it. I even tried another wired mouse and the problem still persists.