aldrincab

const separator = ', ';
const dvType = SpreadsheetApp.DataValidationCriteria.VALUE_IN_RANGE;

function onEdit(e) {
  if (e.value != null && e.oldValue != null && e.value !== "") {
    var dataValidation = e.range.getDataValidation();
    if(dataValidation != null && dataValidation.getCriteriaType() == dvType && 
      e.value.indexOf(separator) < 0 && e.oldValue.indexOf(e.value) < 0) {
        e.range.setValue(e.oldValue + separator + e.value);
    }