Change #1090544 had a related patch set uploaded (by SomeRandomDeveloper; author: SomeRandomDeveloper):

[mediawiki/core@master] Add a warning when trying to create a double redirect

https://gerrit.wikimedia.org/r/1090544

Would it be possible with this to specify, in the error message, the target of the redirect you're attempting to redirect to? The chances are that's what the redirect target should be as well.

In T326056#10334683, @Bugreporter2 wrote:

Would it be possible with this to specify, in the error message, the target of the redirect you're attempting to redirect to? The chances are that's what the redirect target should be as well.

I actually had that in mind and forgot about it again. I could implement this with toEditContent but I'm not sure what the performance impact of that is.

Getting the optimal wording for error messages can be tricky: https://uxwritinghub.com/error-message-examples/

I'd go with something like:

You are trying to redirect this page to a second redirect page, creating a double redirect.  Your target page currently redirects to [[{{{1}}}]].  Please consider revising your target page as follows:
 
<code><nowiki>#REDIRECT[[</nowiki>{{{1}}}]]</code>

Note: if you submit the original information again, the double redirect will be created anyway.

Thanks for the feedback, I was mainly trying to keep it consistent with the other warning messages. After thinking about it again, that didn't really work for conveying more information than just the reason for why it went wrong.

I adjusted the message to what you suggested with a few minor changes, such as displaying the button label like before: