I never buy anything unless it is not really needed.这句中错在哪里,

I never buy anything unless it is really needed.是正确的说法.
句子中的not是多余的.因为never 从不,表示否定.
否定词与until连用,即not until是指直到.才.,除非.才.
原句的意思就是：除非我非常地需要了,我才买东西.