Laravel 5.4 Duplicate entry for key 'PRIMARY' on sync()

I have Playlist and Track Many To Many relationship with additional order field:

Schema::create('playlist_track', function (Blueprint $table) {
    $table->integer('playlist_id')->unsigned();
    $table->integer('track_id')->unsigned();
    $table->integer('order')->unsigned();

    $table->primary(['playlist_id', 'order']);
});

User can delete tracks from playlists and change an order. So this sync method is what I need:

        foreach ( $tracks as $key => $track ){
            $_tracks[ $track ] = [ 'order' => $key ];
        }

        $playlist->tracks()->sync( $_tracks ); 

But I get an error when trying to change an ordering of tracks:

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '3-0' for key 'PRIMARY' (SQL: update `playlist_track` set `order` = 0 where `playlist_id` = 3 and `track_id` = 1)

That's because i'm using composite primary key I guess... But I don't know how to make it works

UPD

This is dd( $_tracks )

array:2 [
  1 => array:1 [
    "order" => 0
  ]
  3 => array:1 [
    "order" => 1
  ]
]