I changed the title and description because this seems to be about reused refs, the "with the same content" part is irrelevant.

Change 562533 had a related patch set uploaded (by Thiemo Kreuz (WMDE); owner: Thiemo Kreuz (WMDE)):
[mediawiki/extensions/Cite@master] Fix bad numbering when reusing sub-references

https://gerrit.wikimedia.org/r/562533

Change 562536 had a related patch set uploaded (by Thiemo Kreuz (WMDE); owner: Thiemo Kreuz (WMDE)):
[mediawiki/extensions/Cite@master] Report conflicting extends="…" with an error message

https://gerrit.wikimedia.org/r/562536

Change 562533 merged by jenkins-bot:
[mediawiki/extensions/Cite@master] Fix bad numbering when reusing sub-references

https://gerrit.wikimedia.org/r/562533

Change 562536 merged by jenkins-bot:
[mediawiki/extensions/Cite@master] Report conflicting extends="…" with an error message

https://gerrit.wikimedia.org/r/562536

Should we consider the task done now with the patch merged? Or what about the comment on the last patch? Should this go into an extra ticket?

We're still not catching this edge case, but the patch makes our feature more complete nonetheless.

  <ref name="a">First</ref>
  <ref name="b"></ref>
  <ref extends="a" name="b">Second</ref>

In T242110#5789469, @WMDE-Fisch wrote:

Should we consider the task done now with the patch merged? Or what about the comment on the last patch? Should this go into an extra ticket?

Let's say this is part of completing the task, since the numbering will be incorrect in the example I gave. It will render as [1] [2] [1.1].

Isn't this the same example as the one in T242141: Bad numbering when reusing a sub-reference before it's defined? To me the example looks more like it is specifically about reusing a sub-ref before it is defined, while this ticket here is about conflicting definitions.

Which means this ticket here is done, doesn't it?

My mistake, what I meant to show was this case:

<ref name="a">First</ref>
<ref name="b">Second</ref>
<ref extends="a" name="b">Second</ref>

This is a conflicting definition in the reverse order.

I'm not sure I understand what the difference is.

<ref name="a">First</ref>
<ref name="b"></ref>
<ref extends="a" name="b">Second</ref>

<ref name="a">First</ref>
<ref name="b">Second</ref>
<ref extends="a" name="b">Second</ref>

In both cases the conflict is the same: The second line creates a top-level ref named "b". The third line attempts to change "b" to be a sub-ref. This is impossible without some kind of multi-pass, which we decided not to do for now. So all we can do at the moment is raise an error when processing the third line. Both should be covered by T242141.