general theory behind 'primary_key' & 'auto_increment' flags, and ON DUPLICATE KEY UPDATE ?

Reading docs.. I want to be sure of things... have your verification of my understanding(s)- Does setting a mysql db column (say "myPrimarykeyID") with the 'primary_key' & 'auto_increment' flags totally negate the logic for ever having to use ON DUPLICATE KEY UPDATE in an INSERT? Or is it that I can do an INSERT with SET myPrimarykeyID=someAlreadyExistingVal, a=1, b=2.... ON DUPLICATE KEY UPDATE a=edited1, b=edited2 ... I am trying to learn/work out the mysql/php code that will allow me to either create *or* edit a record, in one fell swoop. Thanks for any insight! -Govinda