[coyotebush]

> I wonder if they weed out duplicate copies of, say, jQuery.

Linguist tries to filter out things like that:

https://github.com/github/linguist/blob/master/lib/linguist/...

[nox_]

"Tries" is the operative word here. There is a documented history of utter incompetence in Linguist.

[jhcnt]

Likewise for most of GitHub's other libraries. Sundown is some of the worst code I've ever seen.