Is it possible to make double underline in math?
Some equations require that and normaly you wish to emphesize the final result with double underlining it.
underline {underline X} makes a thick underline.
That is the correct way to do it.
The thick underline seems to be a problem with rendering at 100%. If you Zoom at 150% (or higher) you will notice that the two lines have the same thickness.
If you export as PDF it is rendered correctly.
So, yes it is a bug but it is only a display bug 
Thanks for the answer.