Ensure username is unique

app/models/user.rb

@@ -4,6 +4,7 @@ class User < ApplicationRecord
   has_many :candidates
 
   validates :password, presence: true, length: { minimum: 6 }, allow_nil: true
+  validates :name, uniqueness: { case_sensitive: false, message: "already in use" }, allow_nil: false
   validates :email, uniqueness: { case_sensitive: false, message: "already in use" }, allow_nil: true
   validates :invitation_token, presence: true, on: :create
   validate :valid_invitation_token, on: :create