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?

Commencez par la reproduction StackBlitz liée et basculez rapidement entre les onglets Material à l’aide de matTabContent pour observer les contenus dupliqués. Suivez le comportement de lazy-loading impliqué lors des changements d’onglet, puis vérifiez que le fait de quitter rapidement un onglet et d’y revenir ne laisse qu’une seule instance du contenu de chaque onglet.

Rédigé par le modèle d'indexation à partir du texte de l'issue.