function highlightDuplicates() {
  var sheets = SpreadsheetApp.getActive().getSheets();
  var duplicates = {};
  
  for (var i = 0; i < sheets.length; i++) {
    var sheet = sheets[i];
    var data = sheet.getDataRange().getValues();
    
    for (var j = 0; j < data.length; j++) {
      for (var k = 0; k < data[j].length; k++) {
        var cell = data[j][k];
        if (duplicates[cell]) {
          sheet.getRange(j + 1, k + 1).setBackground('yellow');
          duplicates[cell].push([sheet.getSheetName(), j + 1, k + 1]);
        } else if (cell) {
          duplicates[cell] = [[sheet.getSheetName(), j + 1, k + 1]];
        }
      }
    }
  }
}