I have a dirty install of Magento. One of the things I noticed is that e-mails are duplicated for some accounts. This is a quick hack to track them down.

gistfile1.txt

var emails = [];

// Magento 1.7 -- Customer Management Screen
// Make sure to list out all/most of your customers -- or this loses its effectiveness as pagination is now in play.
jQuery('.data tr td:nth-child(4)').each(function() { // Email is in the forth column.
  var email = $(this).text().trim();
  if(jQuery.inArray(email, emails) === -1) {
      emails.push(email);
  } else {
      $(this).css('background-color', 'red'); // Visually find the duplicate.
  }
});