"Cowards die many times before their deaaths" is a figure of speech it does not mean what it says, the meaning is different. Over here it means that cowards die due to their cowardice or in the fear of their sudden death, they die (note, it does not mean physical death)."The valiant never taste of death but once" its again a figure of speech, valiant are dauntless or courageous people simply opposite of coward people. The valiant don't fear death because they know that death is necessary(or have never feared of dying) and "they die once." is their actual physical death.