I had found the issue while working on fixing #33525.

That is if duplicated association has a scope which has `where` with
explicit table name condition (e.g. `where("categories.name": "General")`),
that condition in all duplicated associations will filter the first one
only, other all duplicated associations are not filtered, since
duplicated joins will be aliased except the first one (e.g.
`INNER JOIN "categories" "categories_categorizations"`).

```ruby
class Author < ActiveRecord::Base
  has_many :general_categorizations, -> { joins(:category).where("categories.name": "General") }, class_name: "Categorization"
  has_many :general_posts, through: :general_categorizations, source: :post
end

authors = Author.eager_load(:general_categorizations, :general_posts).to_a
```

Generated eager loading query:

```sql
SELECT "authors"."id" AS t0_r0, ... FROM "authors"

-- `has_many :general_categorizations, -> { joins(:category).where("categories.name": "General") }`
LEFT OUTER JOIN "categorizations" ON "categorizations"."author_id" = "authors"."id"
INNER JOIN "categories" ON "categories"."id" = "categorizations"."category_id" AND "categories"."name" = ?

-- `has_many :general_posts, through: :general_categorizations, source: :post`
---- duplicated `through: :general_categorizations` part
LEFT OUTER JOIN "categorizations" "general_categorizations_authors_join" ON "general_categorizations_authors_join"."author_id" = "authors"."id"
INNER JOIN "categories" "categories_categorizations" ON "categories_categorizations"."id" = "general_categorizations_authors_join"."category_id" AND "categories"."name" = ? -- <-- filtering `"categories"."name" = ?` won't work
---- `source: :post` part
LEFT OUTER JOIN "posts" ON "posts"."id" = "general_categorizations_authors_join"."post_id"
```

Originally eager loading with join scope didn't work before Rails 5.2
(#29413), and duplicated through association with join scope raised a
duplicated alias error before alias tracking is improved in 590b045e.

But now it will potentially be got incorrect result instead of an error,
it is worse than an error.

To fix the issue, it makes eager loading to deduplicate / re-use
duplicated through association if possible, like as `preload`.

```sql
SELECT "authors"."id" AS t0_r0, ... FROM "authors"

-- `has_many :general_categorizations, -> { joins(:category).where("categories.name": "General") }`
LEFT OUTER JOIN "categorizations" ON "categorizations"."author_id" = "authors"."id"
INNER JOIN "categories" ON "categories"."id" = "categorizations"."category_id" AND "categories"."name" = ?

-- `has_many :general_posts, through: :general_categorizations, source: :post`
---- `through: :general_categorizations` part is deduplicated / re-used
LEFT OUTER JOIN "posts" ON "posts"."id" = "categorizations"."post_id"
```

Fixes #32819.