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If I understand you correctly, you are unable to submit the form on your second try, is that correct?
I checked your form and I see that you have set the "Form Limits"->"Unique Question"
This means you won't be able to submit the form is you have already previously submitted a form with the same value. To learn more about this, please check this guide: http://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question
If you want your users to submit the form with repeating values for "Leerlingnummer", then you need to remove it on the "Form Limits".
Do let us know if that resolves the problem.