Follow-up for #3382510: Throw \LogicException when >1 #config_target in the same form targets the same property path

I'm not sure about the error message here, it limits to two, while there could be even more. It is the tiniest of nitpicks though, so setting rtbc regardless.

No notes! +1 RTBC here.

@borisson_ given we're looping through here we can only detect one clash at a time. But once you've removed one - if there is another clash we'll throw another exception.

Committed and pushed 40084a52b03 to 11.x and 23141ce25e6 to 10.2.x. Thanks!

Backported to 10.2.x because this is a bugfix to a feature.

Automatically closed - issue fixed for 2 weeks with no activity.