As an experiment, I went back to Eclipse IDE for Java Developers
Version: Luna Service Release 2 (4.4.2) Build id: 20150219-0600
Using MASON18.jar and then running Ants gives as the picture shows exactly the same thing.
Maybe it is not a problem and is just part of the program as when I increase the display size I usually expand the sim size to the same size as the display frame and you don't see the underlying images.
It was only when I was trying to obtain the border and grid lines and adjust their thicknesses that I expanded one and not the other and saw the repeated images. Lots of others use MASON and may not have tried what I did and thus have not seen this.