中文：我相信人性本善，但是我吃亏了必定以牙还牙，加倍奉还！这就是我的为人原则。 

日文：人の善意は信じまするが、 やられたらやり返す。倍返 しだ！ それが私の流儀なんでね。

today i bought a notebook from Taobao

it is a black cover notebook designed and manufactured in Taiwan.

i love the quotations on every page. wriiten in Japanese.

page 1:

中文：我相信人性本善，但是我吃亏了必定以牙还牙，加倍奉还！这就是我的为人原则。
日文：人の善意は信じますが、 やられたらやり返す。倍返 しだ！ それが私の流儀なでね。
英文：i believe that people are basically good.But if suffered,i would return in two folds.That's my principle.

P.S：it seems like originating from <半沢直樹>

Hello Peter Pan,

Please excuse the long explanation, but I thought the sentence was very interesting and very hard to translate so I became interested in finding a very good translation.  I cannot read chinese, but here is my (fluent english) translation of the japanese sentence.

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I believe that people are basically good, but if made to suffer I return it twofold as a matter of principle.

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--> It's important to use "I" and not "i" for first person

--> Using "But" at the beginning of a sentence is not correct, but it is awkward.  In this situation it is better to use ", but" as a conjunction. 

--> After "," the space is very important.

--> twofold is a single word.  It is very uncommon (it is an 800 year old word used in formal  expressions) but it works very well in this sentence because the sentence is a vow.  Also, twofold is an adverb, not a noun, so it cannot be the object of the preposition "in".  

--> "That's my principle." Is correct english, but the phrase "as a matter of principle" is always used in this kind of sentence.  There is no rule for it, but it is always done this way (colloquial expression).