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array(2) {
  [306]=>
  int(1)
  [149]=>
  int(1)
}

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    $test_array = Array();
    $test_array[306]    = 1;
    $test_array[149]    = 1;
    
    $dupes  = array_keys($test_array, 1);
    
    print_r_pre($dupes);

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Output:

Array
(
    [0] => 306
    [1] => 149
)