Question in title.
Can I cast the "Knighted" attachment on a character that's already a Knight?
Heh heh. "Cast" an attachment.
Seriously, though. Sure. Unless there is something specifically stopping you from playing the attachment on the character, you can do it. The Knight character doesn't really gain anything by getting the trait from the attachment in addition to having it on its own - other than that it's harder to take away - but it does get the +1 STR.
Was there a specific reason you thought you could not play the attachment on the character?