var dsIds = {};
var skip = 0;
var limit = 500;
for (;;) {
  var count = 0;

  print("reading page, skip=" + skip);

  db.dataSources.find().skip(skip).limit(limit).forEach(function(ds) {
    dsId = ds._id;
    var oldVal = dsIds[dsId];
    if (oldVal != null) {
      print("duplicate: " + dsId);
    }
    dsIds[dsId] = 1;
    count = count + 1;
  })
  print("read " + count + " docs");

  skip = skip + count;
  if (count < limit) {
    print("all done");
    break;
  }
}