duplicate 與 copy用法上有什麼分別

Duplicate and copy can both be used as verbs or nouns. They are similar in meaning and different in the ways they are used as below.
As a noun, duplicate is different in meaning from copy. It means a total of two copies (or a additional second copy). (Similarly triplicate means three copies, quadruplicate four copies etc.) We use duplicate when we only want to get a second copy. eg

Can I have a duplicate of the document?
Please issue the invoice in duplicate.
On the other hand, copy (or copies) is used differently. We can use it when we want any number of copies of the document. eg

Can I have a copy of the menu? -- one copy
Please give copies of the answers to every student in the class.--more than one copies
As a verb, duplicate is different in meaning from copy in a similar way. Thus if we want to make another copy of a letter, we can say:

Please duplicate this letter for me.
Copy is used as a verb in a general sense with no specific limitation to how many times something is copied, unless it is otherwise stated. eg

The students copy all the notes from the white-board. -- usually the students copy once.
The teacher asked the students to copy the school rules ten times in the detention class. -- copy ten times as stated

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先講動詞
duplicate 係 複製
copy係 "抄" 或者 照實物弄一個副本

同中文一樣, "複製"係個比較"嚴重"既字, 唔係隨便用得
你可以話你用手抄份文件, 但唔會話你用手"複製"份文件

duplicate係一定要成品同原本一模一樣先可用duplicate
例如: duplicate a computer file, duplicate an image, duplicate a sample, etc. 影印都可以話 make a duplicate.

基本上用得duplicate都可以用copy代替
除左複製人,複製羊果d啦..
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而duplicate同copy都可以係名詞

同上, 要一模一樣的複製品先可以叫duplicate
copy係副本

很多時都可以通用