so-74593543.rb

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  gem 'rails'
  gem 'sqlite3', platform: 'ruby'
end

require 'active_record'
require 'action_controller/railtie'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: 'so74593543')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :companies, force: true do |t|
    t.string :name
    t.timestamps
  end

  create_table :verticals, force: true do |t|
    t.string :name
    t.timestamps
  end

  create_table :company_verticals, force: true do |t|
    t.references :company
    t.references :vertical
    t.timestamps
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Company < ApplicationRecord
  has_many :company_verticals, dependent: :destroy
  has_many :verticals, through: :company_verticals
end

class Vertical < ApplicationRecord
  has_many :company_verticals
  has_many :companies, through: :company_verticals
end

class CompanyVertical < ApplicationRecord
  belongs_to :company
  belongs_to :vertical

  validates_uniqueness_of :vertical_id, scope: :company_id
end

class TestApp < Rails::Application
  secrets.secret_token    = 'secret_token'
  secrets.secret_key_base = 'secret_key_base'

  config.logger = Logger.new($stdout)
  Rails.logger = config.logger

  Rails.application.config.host_authorization = { exclude: ->(request) { true } }
  routes.draw do
    resources :companies
  end
end

class CompaniesController < ActionController::Base
  include Rails.application.routes.url_helpers

  def create
    Rails.logger.debug params.inspect
    company_params = params.require(:company).permit(:name, vertical_ids: [])
    company = Company.create(company_params)
    render inline: company.persisted? ? 'success' : 'failure'
  end
end

require 'minitest/autorun'

class PrimaryCategoriesTest < Minitest::Test
  include Rack::Test::Methods

  def test_index
    Vertical.create(id: 1, name: 'Vertical 1')
    Vertical.create(id: 2, name: 'Vertical 2')
    post '/companies', { company: { name: 'Test', vertical_ids: ["", "1", "2"] } }
    # if you use instead the below line it will trigger the behaviour you're experiencing
    # post '/companies', { company: { name: 'Test', vertical_ids: ["", "1", "2", "2"] } }

    assert_equal "success", last_response.body
  end

  private

  def app
    Rails.application
  end
end