<?php
function helper() {
  $result = array();
  
  foreach (func_get_args() as $fileList) {
    foreach ($fileList as $fileName) {
      list($file, $revision) = explode('[', $fileName, 2);
      $revision = trim($revision, ']');
      $result[$file] = !isset($result[$file]) ? $revision : max($result[$file], $revision);
    }
  }
  
  foreach ($result as $file => $revision) {
    $result[$file] = sprintf('%s[%s]', $file, $revision);
  }
  
  return array_values($result);
}

$a = array('A[1]', 'B[2]', 'C[2]', 'D[2]');
$b = array('B[3]', 'C[4]', 'E[4]', 'F[2]', 'G[2]');

print_r(helper($a, $b));

Array
(
    [0] => A[1]
    [1] => B[3]
    [2] => C[4]
    [3] => D[2]
    [4] => E[4]
    [5] => F[2]
    [6] => G[2]
)