If a Succubus charms a humanoid with the Charm action, and then commands the humanoid to do something suicidal, the target gets to roll a saving throw. Succeeding means they do not perform the suicidal command - it interrupts the command.
But if they fail the saving throw, they go through with the action and take the damage, but should this then result in another saving throw?
If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success.
My interpretation would be that the two are different triggers, even if they happen during the same round. The Succubus gives a suicidal command, the humanoid rolls a saving throw, fail, and then they do the action when it is their turn, taking damage and rolling again.
If not we end up with the awkward situation where falling off a 1000-foot cliff doesn't even give you a chance to save against being charmed, just because you already failed the save for the command itself.