Technical judgement for setting is quite strict. So if the ball slips through your fingers and touches your head it is definitely a double hit. Even if it does not touch your head, the slipping might be enough to be a technical fault (depending on your level/league and the judgement of the referee)
Defense actions are not judged that strict. If the attack is a hard(!) hit it is allowed to touch the ball with two parts of the body (such as in your case foot and chest). However this counts as two actions for your team. So the next player has to hit the ball back over the net.
(The reason behind this rule is to allow for spectacular defense actions. If you keep the ball "alive" after a hard hit, this is what motivates players and what the fans want to see.)