Check Duplicates for Google Sheet

code.gs

var checkSheet = 'Seat';

function onEdit(e) {
  if (e.source.getSheetName() == checkSheet)
    checkDuplicates();
}

function checkDuplicates() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var dataRange = sheet.getDataRange();
  var data = dataRange.getValues();
  var numRows = dataRange.getNumRows();
  var numColumns = dataRange.getNumColumns();
  var formats = dataRange.getBackgrounds();
  var counts = {};
  var modified = false;

  for (var i = 0; i < numRows; i++) {
    for (var j = 0; j < numColumns; j++) {
      if (formats[i][j] == '#ff0000') {
        modified = true;
        formats[i][j] = 'WHITE';
      }
      if (data[i][j] != '') {
        counts[data[i][j]] = 1 + (counts[data[i][j]] || 0);
      }
    }
  }

  for (var i = 0; i < numRows; i++) {
    for (var j = 0; j < numColumns; j++) {
      if (data[i][j] != '' && counts[data[i][j]] > 1) {
        modified = true;
        formats[i][j] = 'RED';
      }
    }
  }

  if (modified) {
    SpreadsheetApp.setActiveSheet(sheet.getSheetByName(checkSheet))
    dataRange.setBackgroundColors(formats);
  }
}