Sum earnings of the same employee

Question

I’m trying to sum up all the earnings of an employee (cumulative sum), but I’m not getting it. The code is following:

function teto() {    
  var ss = SpreadsheetApp; //seta a planilha 
  var planilhaAtiva = ss.getActiveSpreadsheet(); //torna-a ativa
  var abaAtiva = planilhaAtiva.getActiveSheet(); //seta a aba ativa

  for (var i = 2; i < 10; i++) {
    var cpf = [i][3];
    var valor = [i][11];

    if (cpf = cpf+1) {
      valor = valor + (valor + 1);
      abaAtiva.getRange(i,13).setValue(valor); 

    }
  }
}

Explaining: I have a column with numbers grouped from several employees, and another column with the value of the commission q it receives. If the Cpf of a line equals the Cpf of the bottom line, it does the sum. It must make this sum as long as the Cpfs are equal, and put this value in a new column - C, SOMA.
If it is not equal, I do not want it to add up to the previous one, but to start a new sum with the new set of values of the same Cpf.