谁…谁 / 什么…什么 / 哪儿…哪儿【shéi…shéi / shénme…shénme / nǎr…nǎr】

A question word that appears twice, once in each clause, points to the same person, thing or place both times. The first clause sets the condition and the second says what follows for whoever or whatever fits it. The sentence is a statement, not a question.

With 想…就…

想 + question word in the first clause and 就 + the same question word in the second gives "as you like".

Which question words pair

谁, 什么, 哪儿, 哪个, 什么时候, 怎么 and 多少 all work. Each pair keeps the same word; it doesn't switch from 谁 to 哪个 halfway.

The two question words can play different roles. In 谁先到谁买票 both are subjects; in 你喜欢谁就请谁 the first is the object of 喜欢 and the second the object of 请.

The second question word can't be dropped

English says "wherever" once. Chinese needs the question word in both clauses, and a pronoun can't stand in for the second one.

• × 你想去哪儿就去 → 你想去哪儿就去哪儿
• × 你想吃什么就吃它 → 你想吃什么就吃什么

A single question word with 都, as in 什么都吃, means "everything" – that is a different pattern.