A person is not criminally responsible for an offence if it is committed by two or more offenders and if the act is done or omitted only because during the whole of the time in which it is being done or omitted the person is compelled to do or omit to do the act by threats on the part of the other offender or offenders instantly to kill him or her or do him or her grievous bodily harm if he or she refuses; but threats of future injury do not excuse any offence.