title: How to assert that a text only exists 1 time in Mink
body: <p>Im currently working on a small project and i decided to play around with Behat/Mink a little and i came across my first problem i can not solve alone.</p>
  
  <p>I have this feature and it is working as expected</p>
  
  <pre><code>Scenario: Create Customer
    Given I am on "/login"
    When  I fill in "username" with "testuser"
    And   I fill in "password" with "123"
    And   I press "Login"
    And   I follow "Customers"
    And   I follow "Create"
    Then  I should be on "/customer/new"
    And   I fill in "email" with "test@test.de"
    And   I press "Save"
    Then  I should be on "/customer/"
    And   I should see "test@test.de"
  </code></pre>
  
  <p>When i press on "Save" the project then checks, if the email already exists. If it does it just redirects to /customer. How can i assert, that a text is only once (not twice or more) on my page?</p>