Let's say we have two arrays like this:
$threads = array(
'title' => 'Lorem ipsum',
'body' => 'Bla Bla',
'message_id' => 1
);
$replies = array(
'title' => 'Ipsum Lorem',
'content' => 'Bla Bla',
'reply_id' => 2
);
Notice that both arrays have one key called 'title', so we when we merge we one of them will be overridden
$threads->merge($replies);
// Will return
$results = array(
'body' => 'Bla Bla',
'message_id' => 1
'title' => 'Lorem ipsum',
'content' => 'Bla Bla',
'reply_id' => 2
);
As you can see, the key of an array needs to be unique. So probably the two arrays have one field that have the same key value. You need to update the key to prevent this.