A challenge is usually just a public notice of intent to kill. If it is returned, it should be left to the two combatants, with no help. (No CCC's and such) Especially for a knight.
Not all respect the sanctity of a challenge. (Xandamere leaps to mind.)
I give a challenge so that my opponent knows I am going to attack, thus I do not usually \"jump\" so to speak.
That's my etiquette at least. I am sure many will dispute it.
Sir Arthor, for more challenges.
Written by my hand on the 17th of Hindyear, in the year 978.