Return the INDEX of the first duplicate number in an array of number

finddup.rb

# Hello Marc, so I just couldn't let our challenge go. after I left I kept thinking
# about the solution that I provided and the fact that it wouldn't work. So tonight
# I decided to code it and write some test for it and I wanted to share it with you.
# If I copied the array and then deleted the elements the program would always return 0
# because that is where the duplicate number would live at the time it got compaired.
# I would love some feedback and would also love to see how you might solve this.

def find_duplicate_retrun_index(arr)
  arrCopy = arr
  arr.each_with_index do |value1, index1|
    arrCopy.each_with_index do |value2, index2|
      if index1 == index2 || value1 != value2
        arrCopy.delete(0)
      else
        return index1
      end
    end
  end
end

## RSpec file finddup_spec.rb

require_relative 'finddup'

describe 'find_duplicate_retrun_index' do
  let(:numbers) { [1, 2, 3, 4, 5, 5, 6, 7] }
  let(:numbers1) { [1, 2, 3, 4, 5, 8, 6, 10, 10] }
  let(:numbers2) { [7, 1, 2, 3, 4, 5, 8, 6, 7] }

  it "method is defined" do
    expect(defined? find_duplicate_retrun_index).to eq 'method'
  end

  it "method takes a single argument" do
    expect(method(:find_duplicate_retrun_index).arity).to eq 1
  end

  it 'Returns the index of the first duplicate number' do
    expect(find_duplicate_retrun_index(numbers)).to eq 4
  end

  it 'returns the index of the first duplicate number' do
    expect(find_duplicate_retrun_index(numbers1)).to eq 7
  end

  it 'returns the index of the first duplicate number' do
    expect(find_duplicate_retrun_index(numbers2)).to eq 0
  end

end