You could achieve the double line by using \hhline{#=#===#=#} as in
But if you zoom out you'll see some weird details as the one inside the red circle (but visible along the lines):
But as you would read somewhere else, I'd advise against the use of vertical rules to improve readability. Not to mention that you would have a much cleaner code.
So this is my suggestion for your table. It uses the booktabs package which makes for much better lines:
Best Answer
A couple of solutions, with or without vertical rules:
If the cell content is plain text instead of in math mode, just change
L
andC
(the new column types I created) intol
andc
.