>> Suppose we have such customization:
>>
>>    (setq display-buffer-alist '(("\\`\\*grep\\*\\'" display-buffer-same-window)))
>>
>> Now after running grep with a non-empty output in ‘emacs -Q’,
>> typing ‘C-o’ (compilation-display-error) will display two identical
>> buffers instead of displaying the buffer with the found grep hit.
>>
>> The problem is with pop-to-buffer in next-error-no-select.
>> I wonder what is an idiomatic way to ensure a buffer is already
>> displayed in some window, to not display the same buffer
>> in other window?
>
> I'm not quite sure I understand: Is there a reason you did not supply
> 'display-buffer-reuse-window' in the above customization?

Because it's an idiomatic way to tell the Emacs window manager to
display the *grep* buffer in the same window (an old way to do the same
was using same-window-buffer-names and same-window-regexps)
when manually running M-x grep.

But it fails when using C-o from the already displayed *grep*.
I think next-error-no-select should override the user setting with
display-buffer-overriding-action because the purpose of pop-to-buffer in
next-error-no-select is to ensure the *grep* buffer is displayed somewhere: