Fix bug when mutation causes controller to be registered twice

[lewispb]

Partially reverts 67e2cf3#diff-8e0a8f9875453f690405e8c037a8e76c4436f09f47fd9c605283c2f16bd95789L77

Resolves an issue where a controller is registered twice.

I created a sample Rails app to reproduce this issue. Relevant commit: lewispb/stimulus-loader-issue@ce8aaa5

To reproduce:

1. Insert a Turbo Stream element onto the page which in turn appends an element specifying a Stimulus controller
2. It appears the mutation observer triggers twice, once for the HTML element mutating (as the Turbo Stream element is inserted into the dom) and once for the append performed by the Turbo Stream action.
3. This triggers registering the Stimulus controller twice, and that has the effect of immediately disconnecting the controller

Before

The first inserted element is removed as the disconnect function is immediately called. In our application this is a flash message.

before.mov

After

Elements are removed correctly, after the timeout.

after.mov

[dhh]

I take that this is dealing with a race condition when the same controller is listed twice? Because otherwise the existing guards should take care of it. Anyway, all good.

[lewispb]

Yeah, but manually appending multiple elements with the same controller to the DOM didn’t reproduce. Requires turbo stream to reproduce and I ran out of time trying to understand exactly why.