It has never been stated explicitly that the Riku Duel wielded to create the kairi keyblade. While interpretations of vague statements may lead you to believe one thing, just cuz it fits with your interpretation does not make it fact. There is no hard evidence one way or the other, so it is inappropriate to accept that "Riku duel wielded, that's how she got her keyblade" as a hard fact. Until we get a good deal more information, it is all speculation.