[7.x] Fix 'loadCount' method to ensure count is set on all models

[gdebrauwer]

If a model is present multiple times in a collection, all models should get the relationship count set, instead of only the first occurence of that model in the collection.

$posts = Post::where('id', 1)->get()->push(Post::where('id', 1)->get());

$posts[0]->comments_count; // is set
$posts[1]->comments_count; // is not set (bug)

I discovered this bug while I was creating PR #32739 that adds loadMorphCount method. That method uses the loadCount method. The temporary collection that calls the loadCount method, can contain duplicates (e.g. 2 comments can morph to the same post)