Unique Value enforced

Problem: I have a membership table with members listed by name. I want to keep people from typing their information into the membership form on our website that feeds into the table twice. There is no way that I can see for me to set a field to not allow duplicate entries. I would like to set the phone number as the unique identifier for each member. I see no way to select or set this field to not allow duplicates. I know I could that in Access by setting it as primary, but I can’t do that in Airtable.