The Cucumber Book, Second Edition: Chapter 10

Title: Name of book: The Cucumber Book, 2nd ed - Chapter 10
Example: Neither working through the example code nor downloading the code and using that works. The errors I get when running the code I worked through are as follows:

Scenario: Successful withdrawal from an account in credit # features/cash_withdrawal.feature:2
Given my account has been credited with $100 # features/step_definitions/account_steps.rb:1
Validation failed: Number has already been taken (ActiveRecord::RecordInvalid)
./features/support/world_extensions.rb:15:in my_account' ./features/step_definitions/account_steps.rb:2:in /^my account has been credited with $(\d+)$/’
features/cash_withdrawal.feature:3:in `Given my account has been credited with $100’
When I withdraw $20 # features/step_definitions/teller_steps.rb:3
Then $20 should be dispensed # features/step_definitions/cash_slot_steps.rb:1
And the balance of my account should be $80 # features/step_definitions/account_steps.rb:5

Failing Scenarios:
cucumber features/cash_withdrawal.feature:2 # Scenario: Successful withdrawal from an account in credit

1 scenario (1 failed)
4 steps (1 failed, 3 skipped)

I am not a Ruby programmer so I havent a clue what is going on here as I have faithfully replicated the example code in the book. Please help.

Thanks