Don't allow duplicate traits

Description

A factory can have the same trait multiple times

Reproduction Steps

FactoryBot.define do
  factory :user, class: OpenStruct do
    trait :admin do
       name { "Admin" }
    end

    trait :admin do
       name { "Administrator" }
    end
  end
end

Expected behavior

Raises an error when defining the second trait

Actual behavior

Lets me define a second trait but keeps only the first trait

System configuration

factory_bot version: 6.1.0
rails version: 6.1.3.1
ruby version: 2.7.3

Start by tracing how a factory registers a trait during the trait definition entry point, then inspect the existing tests around trait definitions. Add a regression test for defining the same trait name twice and verify that the second definition raises an error rather than being silently ignored.