Adjective: unchanged when multiplied by itself. Used in mathematics and SOA (services oriented architecture) software development.
This could apply to people too! An idempotent person is someone who remains unchanged despite repeated exposure to the same input.
Mr Guts who eats like there is no tomorrow and yet does not gain weight would have an idempotent metabolism. Miss PR who wholeheartedly believes in her own BS would be the opposite of idempotent.
(Ok, so this is a silly post)