从数组中删除重复项

我使用下面的代码行来遍历数据库中的表：

$items_thread = $connection -> fetch_all($sql);

如果我打印出阵列：

print_r($items_thread);

我会得到这个：

Array
(
    [0] => Array
        (
            [RecipientID] => 3
            [RecipientScreenname] => Tom L
            [RecipientFirstname] => Thomas
            [RecipientEmail] => info@xx.com
        )

    [1] => Array
        (
            [RecipientID] => 3
            [RecipientScreenname] => Tom L
            [RecipientFirstname] => Thomas
            [RecipientEmail] => info@xx.com
        )

    [2] => Array
        (
            [RecipientID] => 1
            [RecipientScreenname] => Lau T
            [RecipientFirstname] => TK
            [RecipientEmail] => lau@xx.co.uk
        )

)

但是我想摆脱数组中的重复项，所以我使用array_unique

print_r(array_unique($items_thread));

我得到下面的奇怪结果，这不是我正在寻找的：

Array
(
    [0] => Array
        (
            [RecipientID] => 3
            [RecipientScreenname] => Tom L
            [RecipientFirstname] => Thomas
            [RecipientEmail] => info@xx.com
        )

)

理想情况下，我认为它应该返回这个：

Array
(
    [0] => Array
        (
            [RecipientID] => 3
            [RecipientScreenname] => Tom L
            [RecipientFirstname] => Thomas
            [RecipientEmail] => info@xx.com
        )

    [1] => Array
        (
            [RecipientID] => 1
            [RecipientScreenname] => Lau T
            [RecipientFirstname] => TK
            [RecipientEmail] => lau@xx.co.uk
        )

)

我该怎么做才能把它做好？我是否使用了错误的 PHP 语法/默认函数？