If the same extension is applied twice, a warning (if not an exception) should be thrown

[GuySartorelli]

There's no valid use case for applying the same extension twice to any class, and having an extension applied twice means hooks fire twice which will only ever cause problems.

If an extension is applied, and that extension is already present, a warning or exception should be thrown. Probably an exception.

Things to consider:

1. Is suddenly throwing an exception a breaking change?
   • There's precedent to say this is fine for a minor, see https://docs.silverstripe.org/en/5/changelogs/5.2.0/#react-forms-exception
   • If someone does have some "valid" use case for this scenario (which I don't think is possible but I've been wrong before), throwing an exception would break their use case.
2. Do we check for class inheritance? e.g. I have Versioned applied, I make a MyVersionedSubclass extends Versioned and apply it. Does this throw the same warning or exception?
   • I think it should, since the subclass will have all the same methods the original class does and therefore cause the same problems as if the original class was applied a second time