When my clients fill out the form you get an error "Sorry one entry is allowed. Multiple submissions are disabled for this form."

It appears that your form has a Unique Question set in the preferences. When a unique question is set, you will only be able to accept unique entries based on that unique question. This means that when a user inputs the same data on the unique question, the form will not accept it and they will get the error "Sorry one entry is allowed."

You currently have the "Curso" field set as the unique question, which means duplicate data in the "Curso" field will not be accepted. 

To resolve this issue, please set the Unique Question to "No Check" instead.

For more information on Unique Questions, please check out this guide: How to set Form Limits Based on a Unique Question.

Thanks for the reply .I will check how it works now