Double slashes are usually caused by redirect rules. For example RewriteRule ^(.*)$ http://www.example.com/path/$1 [R=301,L] could cause the double slash. The regular expression's parenthesis might be returning /filename.html for the $1 variable. The two slashes come from one from the match and the one hard coded.
To make this redirect work as expected, ...
So far, I found only two solutions:
(1) Use ħ instead of ℏ - i.e., ħ instead of ℏ
The Firefox development team is working on an improvement as well.
(2) Another option is to use:
which yields the correct size.
Your page appears to use the Google web fonts "Open Sans" and "Crimson Text". Neither of those fonts has a glyph for ℏ as tested by Google's tool:
There are a couple Google fonts that support the ℏ character. Scrolling through I see "Noto" (several serif and sans-serif variants) and "Arimo". If you want this ...