Why does CSS3 duplicate
This duplication confuses, as each is listed separately on the aforementioned CSS3 link.
The X Window System contained a text file that had RGB values that mapped to certain colors. This text file was shipped with all X11 installations.
Once the HTML color list was defined, instead of deprecating the duplicate and clashing colors, they simply adopted them into the list - presumably to prevent breaking older websites/browsers that used X11 colors and because X11 had defined them first.
That being said, if you use a clashing color name on a site today, it would display the W3C color instead of the X11 color.