twin指双胞胎之一,而不是指双胞胎两个,指两者时要用复数.比较：
I can’t tell one twin from the other.我分不出双胞胎中谁是谁.
I spoke to both of the twins this morning.今晨我与两个双胞胎都谈了谈.
类似地,parent指父母之一,即指父亲或母亲,要表示父母双亲,应用复数parents.比较：
A half-brother is brother with only one parent in common with another half-brother指同父异母或同母异父的兄弟.
The parents named their baby Tony.父母给他们的孩子取名托尼.
（摘自《英语常用词多用途词典》twin词条,金盾出版社）