Using Google forms, data can saved to the spreadsheet. Now i would like to apply validation in Username field for avoid duplication. How can add UNIQUE validation in google form field using Google script?
I think it is not possible to add validation for fields in Google form values by using Google Apps Script direcly.
One thing you can consider to change the settings in the Google form itself, if the Username field value is same as the Username that logged in to access that form. You can do this by checking the "only allow one response per person(required login)".
If the above solution does not apply for your requirement, the UiApp/Html Service in Apps Script will help you in creating custom forms.
You can create a custom form and write a custom function to get the Username field values using SpreadsheetApp class into an array. Before submitting the Form, compare the Username field from Google form with the list of username cell values that we got in an array and check for uniqueness.
Hope that helps!
来源：https://stackoverflow.com/questions/26708091/unique-field-validation-in-google-forms