Google Apps Script: trigger only for new users
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I have script which creates a new sheet in a google spreadsheet under the names of the users who fill out the form of the spreadsheet. 

I would like this script to be triggered whenever a new user fills out the form associated with this spreadsheet. of course, a new user means a new E-mail address is added to the automatically generated "Username" column in the spreadsheet.

This is my code currently:

    function MakeSheet() { 
    var ss = SpreadsheetApp.getActiveSpreadsheet(); 
    var sheet = ss.getSheetByName("default sheet"); 
    var getEmail = Browser.inputBox("user Email: "); 
    var s1 = ss.getSheetByName("list of users and emails"); 
    var index = s1.getRange(1,1,s1.getLastRow(),1).getValues(); 
    var values = s1.getRange(1,2,s1.getLastRow(),1).getValues(); 
    nextIndex: for( var i in index) { if( index[i][0] == getEmail ) { var getName =   values[i][0];continue nextIndex;}}

    ss.setActiveSheet(ss.getSheetByName("default sheet"));
    SpreadsheetApp.getActiveSpreadsheet().duplicateActiveSheet();
    ss.setActiveSheet(ss.getSheetByName("Copy of default sheet"));
    SpreadsheetApp.getActiveSpreadsheet().renameActiveSheet(getName);
    SpreadsheetApp.getActiveSheet().getRange('B3').setValue(getEmail);
    SpreadsheetApp.getActiveSheet().getRange('C3').setValue(getName);
   
    }