I am trying to create a fill-able form in Word as a template for an internal investigation report. Since the number of complainants, respondents and witnesses are different for each investigation, I would like to know whether Word has a function to duplicate a set of form fields with click of a button (e.g., name, address, phone, etc.). Thank you.

Not exactly the click of a button, but:

• AutoText / Building Blocks Automated Boilerplate Using Microsoft Word
• Repeating Section Content Control

Look into changing from legacy formfields to Content Controls and not using the overly restrictive "filling in forms" editing restriction. Instead, you could use a Group Content Control or No Changes / with exceptions.