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]]; } } } } }