They are not necessarily the same.
RPG.SE meta's favicon is a gray d20 outline. I'm not able to upload the .ico file here, but it has a MD5 hash c7f81da9029121051950ae912546a092.
The engl DND5 wikia's favicon is a black d20 outline. The .ico file has a MD5 hash 82f5514407b95ad867762108f1e02953. This means it's a different file.
(For those who want to check sources: Go to either site, view page source, find "shortcut icon" in the page header HTML, and download the file from the URI. Then check the file's MD5 hash.)
Also, on their front page, they have the following disclaimer:
d20 dice favicon was created by Andrew Doane from the Noun Project and is licensed under the Creative Commons CCBY.
Is it possible that the wikia owners copied RPG.SE meta's favicon, changed the color, and then used it for themselves? Yes. But we have no proof.
It is entirely possible that, like this site, they decided to use a d20 icon because their site features information about a d20 system. RPG.SE doesn't have a copyright on the d20 outline design. It's a minimalistic design and, if you image search for "d20 icon", you will find that most of the results have a similar appearance.