Using related records from parent across 3 fields in 2 different tables

I've driven myself more or less crazy trying to figure this out on my own. The relationships themselves are rather simple: Each project has a list of questions. Each question has a person who created the question, and someone the question is assigned to. A question also has a number of responses associated with it which each have their own "creator".

There are 3 main tables to cover: Questions, People, and Responses

• People consists of 3 fields: The UID, the person's name, and the person's email address
• Questions has 2 fields which relate to People: creatorID and assigneeID
• Responses has 1 field which relates to People: creatorID.

The way the relationship graph is set up now, I have 3 instances of the People table that relate the UID field to the appropriate ID field in Questions or Responses as needed.

What I want to happen is when I set an Assignee or Creator for a question or response, I want to use the corresponding record details from the People table - if one does not exist, then create a new one. Instead what happens is a get a new record in the people table every time. Even if I set the Layout field to give me a drop down of the existing values (and I use it to select one when, say, setting the assignee on a question) it still creates a new record. So confused... any help is much appreciated.