I haven't noticed before, but it seems that mReferencesErrors was previously only used for errors that appeared from <ref> tags inside a <references> tag, and the errors stored there were only rendered when the <references> tag was being rendered. Since we used that in this patch, the error doesn't appear if <references> is missing.
There's also another interesting effect: when using grouped refs (<ref group>), the error message will appear after the first <references> tag (regardless of its group), and not the one where the duplicate ref belongs.
It should be possible to move the error message to be displayed next to the text of the problematic reference, rather than at the end of <references> block.
I copied the contents of https://www.mediawiki.org/wiki/User:IKhitron/draft to my local wiki, here's a screenshot of what my change does:
|Local wiki with patch 244194|
The wikitext of this is:
<ref group=q name=a>gjgjjhgh</ref> <ref group=q name=a>gjjjhgh</ref> <ref name=c/> <ref name=a>jkgkgkgkg</ref> <ref name=a>jkgkgkggkg</ref> <ref name=a>jkgkggkg</ref> <ref name=a>jkgkggkgfgh</ref> <references/> jhgkgh <references group=q/>