Howto update duplicates in firebird?

Searched google for "firebird update duplicates" and "firebird handle duplicate update" but did not get any stuff, that would solve my question; so i thought it would be a good idea to use this marvellous site for the first time :) So how do i achieve mysql's "INSERT INT开发者_如何学编程O asdf (x,y,z) VALUES ('a',1,2) ON DUPLICATE KEYS UPDATE z = 2" in firebird?

thanks in advance!

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UPDATE OR INSERT INTO <table or view> [(<column_list>)]
    VALUES (<value_list>)
    [MATCHING (<column_list>)]
    [RETURNING <column_list> [INTO <variable_list>]]

Update or Insert