精益求精, how to properly interpret this classical Chinese chengyu?
精 http://www.cantonese.sheik.co.uk/dictionary/characters/565/
[4] [adj] skilled; versed; proficient

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益 http://www.cantonese.sheik.co.uk/dictionary/characters/1217/
[1] [v] increase; add to; augment [2] in a higher degree; to a greater extent; more

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求 http://www.cantonese.sheik.co.uk/dictionary/characters/883/
[2] seek

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精益求精 http://www.cantonese.sheik.co.uk/dictionary/words/7244/
(idiom) to constantly improve oneself

Can someone please better explain the arrangement XY求X?

It is an idiom, not a sentence. Idioms do not have to follow normal grammar rules. You just have to look at each character within an idiom to construct a coherent meaning in your mind. In the case of 精益求精, it offers:
精 &quot;proficient&quot; (adj)
益 &quot;in a higher degree&quot; (adverb phrase) +  求 &quot;seek&quot; (V)
精 &quot;proficient&quot; (adj)
(and it does not constitute a sentence. )

&quot;proficient in a higher degree seeking proficient&quot;

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You have to decode the meaning with logical deduction and get:

&quot;(someone already) proficient (still) seeking proficiency in a higher degree&quot;

But most of the time, it is best to learn the story of the idiom and memorize the meaning behind it.