Missing guard for double-require invariant

[Schoonology]

TL;DR: I propose we create a simple guard around doubly-including React, which can cause issues if different blocks work with different instances of React.

I understand this invariant isn't going to matter to everyone, but I've at two points in so many weeks fixed a bug on a co-worker's project because they were accidentally including React twice in the browser: one from a Browserify bundle, and once from a standalone <script>.

[jimfb]

Duplicate of #2402

TLDR: It's more complicated than you might think, but something we want to do eventually.

[Schoonology]

Thanks. Sorry for the dupe, I promise I searched as much as I could think
of first.

On Fri, Jul 10, 2015 at 3:02 PM, Jim notifications@github.com wrote:

Closed #4343 #4343.

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