gistfile1.js

function findDup(r){
  var sheet = SpreadsheetApp.getActiveSheet();
  var dataRange = sheet.getRange(r, 6);
  var data = dataRange.getValues();
  var target = data[0][0];
  if ( target === "" ) {
    return;
  }
  Logger.log(data.length);
  Logger.log(target);
  
  var namesRange = sheet.getRange("names");
  var names = namesRange.getValues();
  Logger.log(names.length);
  Logger.log(names[1][0]);
  
  var foundIn = 0;
  var colors = namesRange.getBackgroundColors();
  for (var i=0; i < names.length ; i++) {
    colors[i][0]="white";
    if( names[i][0] === target ){
      Logger.log("found at "+i);
      foundIn += 1;
      colors[i][0]="#C0FFC0";
    }
  }
  
  if( foundIn > 1 ){
    namesRange.setBackgroundColors(colors);
  }

  SpreadsheetApp.flush(); 
}