From my answer to this question: "https://chinese.stackexchange.com/questions/31120/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 "