Quick function to identify potentially duplicate votes

vote_counting.php

<?php

$votes = [];
$vote_hashes = [];

// time in seconds that must have elapsed between votes
$acceptable_time_delta = 120;

foreach ($voting_data as $row) {

  $count_this_vote = true;

  // generate a unique hash for this vote
  $vote_hash = md5($row['user_agent'].$row['ip'].$row['value']);

  // get the timestamp
  $vote_timestamp = DateTime::createFromFormat('Y-m-d H:i:s', $row['date_created'])->getTimestamp();

  // skip check if this is the first occurence of this vote
  if (in_array($vote_hash,array_keys($vote_hashes))) {

    // delta in seconds between this vote and the previous vote for this hash
    $delta = $vote_timestamp - $vote_hashes[$vote_hash];

    // don't count the vote if it's too soon
    if ($delta < $acceptable_time_delta) {
      $count_this_vote = false;
    }

  }

  // store our vote in our global hashes array
  $vote_hashes[$vote_hash] = $vote_timestamp;

  if ($count_this_vote) {
    $votes[$row['value']] += 1;
  }

}

print_r($votes);