I think having only one copy of a work is the better solution—otherwise, there is no way to say it’s the same thing. Not to mention having two copies means we’d have to manually keep them in sync (or alternatively, have inconsistent data).
Not to mention, having one work gets you the right answers to most questions: “what movements are part of this work?” works either way; “what work is this movement a part of” only gets you the correct answer (both) if they share the work.
If all those Introituses are actually the same work, I think they ought to be merged. But that definitely needs to be investigated first, for each completion—a merge is pretty easy, undoing a mistaken one is not.
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This raises a number of “interesting” questions. One thought is whether the parent works of that work should have the same name with the distinction between them being restricted to the “disambiguation”. My initial feeling (assuming the parents are essentially the same, just differing in the details of the completions of component parts) is that using a common name with disambiguation has the benefit of clarity and stylistic consistency. So, for the Horn Concerto Concerto “Concerto for Horn no. 1 in D major, K. 386b / 412 / KV 514 (Süßmayr completion)” - MusicBrainz, the words “(Süßmayr completion)” would be moved into the disambiguation. This would also be consistent with the approach taken for Mozart’s Requiem.
Secondly, regarding the Requiem, it sounds like a bit of academic study would be required to determine which, if any, of the Introituses (Introiti?) are actually the same - and indeed whether any other parts are also similar.
Thirdly, it would help if a consistent approach to naming composers were used. In the Horn Concerto, I think the Süßmayr completion is described correctly in this respect: Süßmayr is given the credit only for the 2nd movement, not the parent work (where only Mozart’s name is given). For the “catch-all for unknown completions” Concerto “Concerto for Horn no. 1 in D major, K. 386b / 412 / KV 514” - MusicBrainz, however, [unknown] is credited as “additional composer” for the whole work as well as for the second movement. This seems slightly misleading to me, since the first movement was entirely by Mozart. That said, I note that the various revisions of the Requiem seem to adopt different approaches again (for the top-level work): sometimes the completer/reviser is credited as “additional composer”, sometimes as “orchestrator” and sometimes not all. The parts also differ in their accreditation, which seems to reflect the amount of research that may have been done. For example, the Süßmayr completion has him credited as “additional composer” and “additional orchestrator” whereas the Introitus has him credited as “additional composer” and “orchestrator”, also crediting Franz Jakob Freystädtler as “orchestrator”. This counters my (perhaps naive) assumption that the Introiti were similar as it was the only part Mozart completed. Interestingly the Dies Irae Work “Requiem in D minor, K. 626: III. Sequenz: a. Dies irae” - MusicBrainz is credited thus: “additional composer”: Joseph Leopold Eybler, “orchestrator”: Franz Xaver Süßmayr. Getting the correct accreditations for the Requiem is by no means simple! But I am tempted to conclude that adding broad-brush accreditions (other than Mozart himself, of course) at the parent work level, as has been done for the “Süßmayr completion” is not a great idea - these should be done part by part.
Finally, the question arises as to when “Revision of” should be used. This has been done for the Requiem and makes good sense there. However, it has not been done for the Horn Concerto. This may be because the original (incomplete) version of the Rondo is not distinguishable from the whole (Süßmayr completion), but that raises the question as to whether there are in fact any other completions at all and whether the “catch-all” shouldn’t just be deleted.
Little did I realise where my innocent question:
would lead!
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Any chance of a comment from our style leader @reosarevok on the above? Particularly re keeping the distinguishing text in disambiguation and only including the composer for the top work where he/she is involved in all the parts?
I’d generally prefer completion stuff to be in the disambiguation myself, yes.
Re: adding the composer to the parts only, I was going to give Hexaméron as an example where I’d expect no composers… only to find out they’re all there and I added them myself, duh. Collaborative works where each composer only works on one part or two exclusively are the ones where I wouldn’t add anything to the top level, anyway. Something like a completion I’d expect to see listed under the completer (completist? 
), even if any specific parts they didn’t touch don’t include them.
That said, for stuff like Mozart’s Requiem, @ListMyCDs.com has done much more research than I have, so I’d generally defer to either him or someone else who has more experience with this than I do.