Find Duplicate Rows in Google Sheets

There are many ways to find Duplicate Records in Google Sheets. You can do it manually using various spreadsheet formulas or you can use Google Apps Script.

This script, uploaded by Carl Kranich to the Google Drive Template Directory, finds duplicate rows in the active sheet and colors them red but unlike other methods, here you have the option to find duplicates based on data of specific columns.

For instance, if the first column is name and the second is age, you can set the value of CHECK_COLUMNS array as 1,2 and the script will only use these 2 columns to catch the duplicate entries. The columns may be contiguous or noncontiguous.

function findDuplicates() {
  // List the columns you want to check by number (A = 1)
  var CHECK_COLUMNS = [2,3,5,6];
    
  // Get the active sheet and info about it
  var sourceSheet = SpreadsheetApp.getActiveSheet();
  var numRows = sourceSheet.getLastRow();
  var numCols = sourceSheet.getLastColumn();

  // Create the temporary working sheet
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var newSheet = ss.insertSheet("FindDupes");
  
  // Copy the desired rows to the FindDupes sheet
  for (var i = 0; i < CHECK_COLUMNS.length; i++) {
    var sourceRange = sourceSheet.getRange(1,CHECK_COLUMNS[i],numRows);
    var nextCol = newSheet.getLastColumn() + 1;
    sourceRange.copyTo(newSheet.getRange(1,nextCol,numRows));
  }
  
  // Find duplicates in the FindDupes sheet and color them in the main sheet
  var dupes = false;
  var data = newSheet.getDataRange().getValues();
  for (i = 1; i < data.length - 1; i++) {
    for (j = i+1; j < data.length; j++) {
      if  (data[i].join() == data[j].join()) {
        dupes = true;
        sourceSheet.getRange(i+1,1,1,numCols).setBackground("red");
        sourceSheet.getRange(j+1,1,1,numCols).setBackground("red");
      }
    }
  }

  // Remove the FindDupes temporary sheet
  ss.deleteSheet(newSheet);
  
  // Alert the user with the results
  if (dupes) {
    Browser.msgBox("Possible duplicate(s) found and colored red.");
  } else {
    Browser.msgBox("No duplicates found.");
  }
};