The proper spelling is still "amateur." The word was first borrowed from French in the late 18th century, but it still retains the "-eur" ending to this day. The spelling "amature" seems to be a fairly common misspelling of the word, and if you do not know how to spell it, it is a reasonable guess at the spelling, since there are many other words in English that do end in "-ure," such as "future."
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