PHP - Multidimensional Array check for duplicates

I create this multidimensional array:

array (
  0 => 
  array (
    'OrderDate' => '02.11.2018',
    'ClientNumber' => 5500
  )
)

array (
  0 => 
  array (
    'OrderDate' => '02.11.2018',
    'ClientNumber' => 5500
  )
)

array (
  0 => 
  array (
    'OrderDate' => '05.03.2018',
    'ClientNumber' => 5500
  )
)

array (
  0 => 
  array (
    'OrderDate' => '10.12.2018',
    'ClientNumber' => 2200
  )
)

array (
  0 => 
  array (
    'OrderDate' => '10.12.2018',
    'ClientNumber' => 2200
  )
)

The array shouldn't contain duplicate OrderDate's from each ClientNumber.

I want the output to look like this:

array (
  0 => 
  array (
    'OrderDate' => '02.11.2018',
    'ClientNumber' => 5500
  )
)

array (
  0 => 
  array (
    'OrderDate' => '05.03.2018',
    'ClientNumber' => 5500
  )
)

array (
  0 => 
  array (
    'OrderDate' => '10.12.2018',
    'ClientNumber' => 2200
  )
)

What if-condition should i use to make my output look like this? Any help will be appreciated, thank you!

This is my function that is called in a foreach. I tried my best to check with in_array and array_column but the output didn't look like the code above.

$array = array();
function createArray($clientNr, $orderDate){
 if(empty($array)){ // if array is empty, just add
  $array[] = array(
              "OrderDate"=>$orderDate,
              "ClientNumber"=>$clientNr
                 );
 } else {
    if(??????)<--------------------------- // check here if date from clientnumber exists
      $array[] = array(
                   "OrderDate"=>$orderDate,
                   "ClientNumber"=>$clientNr
                  );
    }
 }
}