What’s the difference? I saw this sentence and was confused why there’s two 问s：
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From my answer to this question: "What is the purpose of verb reduplication?":
phrases like 笑笑，看看，走走 or 吃吃 are verb reduplication, it serves to indicate the verb is in a slight degree or a casual manner
The fundamental meaning of the sentence is not change by the verb reduplication, but the tone of the sentence is changed by it:
等他回来的时候问他。- "ask him when he comes back" (level or blunt tone; in a assertive, commanding or requesting manner)
等他回来的时候问问他。- "ask him when he comes back" (softer tone, less assertive, commanding or requesting)
Reduplicate the verb 问 here is like adding filler word like "try" or "let's" in English sentences to make them sound less assertive. e.g. "(try) ask him when he comes back" ; " (let's) ask him when he comes back "