一般無二 Cast From The Same Mould
他的鈴兒怎麼與我的鈴兒就一般無二？縱然是一個模子鑄的，好道打磨不到，也有多個瘢兒，少個蒂兒，卻怎麼這等一毫不差？」他的铃儿怎么与我的铃儿就一般无二？纵然是一个模子铸的，好道打磨不到，也有多个瘢儿，少个蒂儿，却怎么这等一毫不差？」
逐句解 Line by line
"How is it that his bells and my bells are the same in every detail?
一般無二 — alike with no second difference — is the exact phrase the narrator used 2,266 characters ago, when Wukong made the forgeries. The fiend has walked into the description without knowing it.
"Even if they were cast from one mould, surely the finishing would not match,
模 is mú, a mould or template — and this is one of the rare places where the automatic reading gets it right, having twice needed correcting at 模樣 in this same chapter. 好道 is surely, one would think.
"and one would have a blemish more or a nub less,
Two four-character clauses of foundry detail. 瘢 is a scar or casting flaw; 蒂 is the little stub where something was joined. He is arguing like a man who has handled cast metal.
"so how are they alike to a hair?"
一毫不差 is not off by one fine hair — a stock phrase, and unusually pointed here, since the bells he is looking at are literally made of hair. 一 takes yì before the level tone of 毫.
This is the best reasoning anybody does in the chapter and it is completely correct. Two objects that look identical cannot both be genuine; even from a single mould, castings differ in the finishing — a blemish here, a missing nub there. The fiend knows metal, states the case clearly, and draws no conclusion at all.
The joke is in the phrase he ends on. 一毫不差 means not off by a single fine hair, and it is an ordinary idiom for exactness. The bells in front of him are made of Wukong's hair. The text does not point at it.
一般無二 in the first line is the narrator's own phrase from 2,266 characters back, when the forgeries were made, handed now to the victim to say about them. The chapter keeps doing this — giving a character somebody else's words without telling him whose they are.