Why & what caused my checkboxes to visually change?

[wisemank]

Here is one to chew on, and if you have seen this and or even resolved this, maybe you can help.

I have 7 hidden worksheets that have a checkbox control for visibility (on/off).

Why is it, or would it be; that when checked the actual size of the check box gets smaller and graphically the contents and text appear to double each click.... After a few clicks the check box appears to be filled in (black)....?

Anyone seen this or faced this issue? if so how would I correct it permanantly?

Thanks in advanced.....