A large part of the Meta is decided by the player community. 'No duplicates' is the only way to prevent the escalation of unwarranted meta turning into something like 'party looking for 2 war, 2 whm, 8 rdm,' etc.
'No duplicates' allow for the diversity of the playerbase. Other MMOs (i'm looking at you, ESO) got ruined by the meta in that it's always preferred to have a group of magic classes over melee classes, for example.