In the sentence: 漢字は全部書けるようになる必要はないが、たくさん書けた方がいい。
the particle は is used twice in this sentence (漢字は,必要は). Is there any reason why we don't use 漢字を at the start of the sentence here, or is it just the same meaning with slight subtleties.
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The effect certainly will not be the same if you replace は with を, but it only shows in context with other sentences.
You use 漢字は because you want to topicalize 漢字. Two は in one sentence might make it slightly weird, but the author might think the downside is worth it, because (for example) it makes it clear that the sentence is a topic sentence of the paragraph that's followed by a few more sentences about kanji.
By the way, I might say the second は is contrastive, at least in its function, as in 全部は / たくさん ("not all, but many kanji").