This issue just started to happen yesterday. It should be very easy to reproduce. It only happens in firefox v63, which was just updated on my PC yesterday.
Please click to view video demonstration that displays the issue (YouTube). I've posted this sample page online so it can be viewed here: https://demoqa.eadoptonline.com/checkboxtest.aspx
If you use firefox < v63 it will display normally, if you have firefox v63 (current latest release as of this posting) each checkbox is duplicated.
Relevant code to reproduce the issue:
Code behind is unimportant
The form decorator in my master page is as follows. The skin comes from web.config but it doesn't matter what i use the problem appears tied to the render mode, not the skin.