"We did not finish eating this Peking duck."
It is the quantifier for Peking duck, e.g. 一只鸡、一盘面、一碗饭、伊梨苹果 and so on, different quantifiers for different nouns just as it is in English.
Only a small percentage of English words need quantifiers and something like Peking duck would not be one of them. Probably most English speakers who haven't studied an Asian language would not even know that English has them.
So saying "just as it is in English" could confused some people.