http://www.cardgamedb.com/index.php/legendofthe5rings/legend_of_the_five_rings

Quote

Action: During a conflict, if you control a participating Courtier character, choose a participating character controlled by your opponent. Your opponent must select one – dishonor the chosen character or bow it.

If you target a character already dishonored may your opponent select dishonor him (¿again?) or he must choose to bow him?

Per the Online Rules Reference, under "Select":

◊ For all selections, an option that has the potential to change
the game state must be chosen, if able.

Therefore, if the target of For Shame! is already dishonored, it must be bowed. Or if it is already bowed, it must be dishonored.

3 hours ago, Zesu Shadaban said:

Therefore, if the target of For Shame! is already dishonored, it must be bowed. Or if it is already bowed, it must be dishonored.

And a character that is both bowed and dishonored is not a legal target.