array_column() will return duplicate values. 
Instead of having to use array_unique(), use the $index_key as a hack.
**Caution: This may get messy when setting the $column_key and/or $index_key as integers.**
<?php
$records = [
 [ 'id' => 2135, 'first_name' => 'John' ],
 [ 'id' => 3245, 'first_name' => 'Sally' ],
 [ 'id' => 5342, 'first_name' => 'Jane' ],
 [ 'id' => 5623, 'first_name' => 'Peter' ],
 [ 'id' => 6982, 'first_name' => 'Sally' ]
];
print_r(array_unique(array_column($records, 'first_name')));
// Force uniqueness by making the key the value.
print_r(array_column($records, 'first_name', 'first_name'));
print_r(array_column($records, 'id', 'first_name'));
// Returns
/*
Array
(
 [0] => John
 [1] => Sally
 [2] => Jane
 [3] => Peter
)
Array
(
 [John] => John
 [Sally] => Sally
 [Jane] => Jane
 [Peter] => Peter
)
Array
(
 [John] => 2135
 [Sally] => 6982
 [Jane] => 5342
 [Peter] => 5623
)
*/
?>