function fillActiveSheet(){
var sheet = SpreadsheetApp.getActiveSheet();
var dataRange = sheet.getDataRange();
var data = dataRange.getValues();
var headers = data.shift();
var newData = fillData_(data);
newData.unshift(headers);
dataRange.setValues(newData);
}

function fillFullActiveDataRange(){
var range = SpreadsheetApp.getActiveRange();
range.setValues(fillData_(range.getValues()));
}

function fillData_(data){
return data.map(function(row){
return row.map(function(item, i){
if(item !== this[i] && item !== '')
this[i] = item;
if(this[i] === undefined)
this[i] = '';
return this[i];
}, this);
}, data.slice(0, 1));
}