Difference between 问问 and 问
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 "