Fixes bug if there are duplicate posts in items array

[ajoah]

Hi,

If i try to connect posts from array which contains same post multiple time, only the first post is connected.

Exemple :

$posts = [
object(WP_Post) { 
 ['ID'] => 1,
 [...]
},
object(WP_Post) { 
 ['ID'] => 1,
 [...]
}
]
p2p_type( 'movies_to_actors' )->each_connected( $posts, array(), 'actors' );

var_dump($posts)  => [
object(WP_Post) { 
 ['ID'] => 1,
 ['actors'] => [...]
 [...]
},
object(WP_Post) { 
 ['ID'] => 1,
 [...]
}
]

The second post should have an actors attribute.

This pull request fixes this bug.