The rules forum is probably a better venue for questions such as this.
Can a character stand itself if it is already standing? For example, can the Red Viper (the one that raises claim) activate the challenges action by taking a gold off of him to "stand him" when he's already standing, then discard another gold off of him to raise the claim?
This is kind of a "yes and no" answer.
Yes - it is legal to attempt to stand a character that is already standing, or kneel a card that is already kneeling.
No - Such an attempt is considered to be unsuccessful. So anything that requires that standing or kneeling to work first (like The Red Viper standing before he can raise claim) will be unsuccessful as well.