Material Tabs with lazy loading rendering duplicate content

#### Bug, feature request, or proposal:
Material tabs (with `matTabContent`) are rendering multiple instances of the tab body inside the same tab when switching away and back too fast.

#### What is the expected behavior?
Not seeing duplicate content.

#### What is the current behavior?
There are multiple instances of the same body/content inside.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-jwxe85
Switch back and forth quickly on these tabs and pay attention to what happens to the one in the middle. You will notice content being rendered multiple times.

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

#### Is there anything else we should know?

リンクされた StackBlitz の再現から始め、matTabContent を使用して Material タブをすばやく切り替え、重複した本文を観察します。タブの切り替えに関係する lazy-loading の動作を追跡し、すばやく別のタブに切り替えて戻ったときに、各タブの本文のインスタンスが 1 つだけ残ることを確認します。

索引モデルが issue の本文から書いたものです。