Hi @Cielquiparle, thanks for taking the time to report this! Does this also happen in safemode with zero gadgets or user scripts involved?

Yes! I switched to safemode and was able to recreate the problem just now:

(Though I wasn't sure if I need to do something else to disable all my gadgets besides viewing in safemode? A further detail is that I use the "Timeless" skin by default, which I think sometimes behaves differently from other skins, but maybe it's not relevant.)

This may be fixed this week by https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Cite/+/755739.

Looks promising, it definitely had something to do with the Reference numbers not updating / shifting properly,

In fact, come to think of it, what seemed to be triggering the problem initially was when I would try to Re-use Citations that had occurred lower down on the page and insert them much higher up on the page. (But now I think that's not the only time it happens.)

Not sure if the fix has gone live yet...but I just encountered this problem again. (I was also wondering if it's likely that the fix will prevent the problem moving forward but won't be able to address existing instances of errors...?)

In T300066#7658097, @Cielquiparle wrote:

Not sure if the fix has gone live yet...but I just encountered this problem again.

I'm not sure if it was live at the time you tested, but it definitely is now, if you'd like to re-test. (Note that we have one other significant known issue related to re-using references: T299378.)

(I was also wondering if it's likely that the fix will prevent the problem moving forward but won't be able to address existing instances of errors...?)

Yes, if a page was already saved with the error, it won't be changed until the page is edited and the reference definition is corrected.

Still happening. Specifically:

• When you insert / re-use an existing citation higher up on the page than where it was first inserted/created, and edit it (for example, by adding page numbers) using Visual Editor, you get this error:

• The error states that "The named reference ":NUMBER" was defined multiple times with different content."

@Cielquiparle I'm sorry for being late to respond. I tried and I can't reproduce the problem.

It looks like you were editing the page https://en.wikipedia.org/wiki/Harriet_Robinson_Scott. Here's a screen recording that shows me re-using the citation that way without problems:
(better quality video: https://drive.google.com/file/d/1oP88f6Cl6PqabggDxv1uV3f79kwtH4bG/view – Phabricator won't let me upload it here)

In the video, I go to the first use of the citation, re-use it above (by copy-pasting), change it, confirm it looks fine in visual editor, preview the wikitext diff and switch to wikitext editor, and confirm that it looks fine in the wikitext editor preview. I also didn't see any issues when it's a newly inserted citation, or when re-using using the toolbar menu (not shown in the recording).

Are you doing something differently?

As far as I know, this is fixed. Handing over to QA to double-check.

@Cielquiparle Please let us know if you still see the issue.

In T300066#7839305, @matmarex wrote:

As far as I know, this is fixed. Handing over to QA to double-check.

@Cielquiparle Please let us know if you still see the issue.

The issue must have been fixed as it works fine on my end as well. Thanks for the attached video @matmarex.

For visual reference, see T300066#7723733.