Firestore: ensure mixin being applied no more than once.

[merlinnot]

Inexperienced users may apply FirestoreMixin more than once to the same element. In that case, we can have some nasty race conditions and errors like doubled values etc.

[merlinnot]

I came to a conclusion that it's better to handle this use case rather than throw an error. It can be useful if a user has multiple mixins relying on FirestoreMixin.