Hit the ball twice: protecting the wicket is a limited exception
Why a deliberate second strike may be legal for defence but not for scoring.
Not every second contact is an offence
MCC Law 34 concerns the striker wilfully striking the ball again before it has touched a fielder, while the ball remains in play. The initial contact can be with the bat or the striker's person. This is not a shortcut that makes every accidental two-contact shot a dismissal.
A deliberate second stroke solely to guard the wicket has a specific exception. Before a fielder touches the ball, the striker may use the bat or body, but not a hand that is not holding the bat, for that limited purpose.
Defence does not permit stopping a catch
The exception does not allow the striker to prevent a catch by making a second strike in defence of the wicket. Law 34 points to the obstruction provisions for that case. No-ball status also needs separate attention; it does not simply erase every second-strike rule.
Consider a ball rolling back towards the stumps after a shot. A lawful defensive tap is different from deliberately hitting it again to add distance and run. The purpose and sequence matter.
Lawful defence is not another scoring shot
For a lawful second strike, the Law sets a dead-ball procedure and disallows running scores, with specified treatment of no-ball and penalty awards. Do not treat a ball sent to the rope after that defensive tap as an ordinary four.
The bowler does not receive wicket credit for a hit-the-ball-twice dismissal. The team wicket total and the bowler's personal wicket count can therefore change differently without the scorecard being wrong.