Translation:We only want a half order of Peking duck.
They are two differemt words (with two different tones), although the simplified characters are the same.
The 只 before 要 is zhǐ, and means "only." The 只 after 半 is zhī, and a measure word for some kinds of animals.
I see only one instance of 半, but in case you are wondering why 只 appears twice, it's because the second 只 is the simplified form of 隻 and is used as a classifier for 鸭
Until now 要 has always meant to need (although I have always used it as want). Whenever there has been want in previous exercises it has been 想.