You’re far more knowledgeable about music notation than I am, so I’ll happily defer to your expertise. That said, is it ever the case that slurs would preferably connect two identical notes, when there is a clear alternative in another voice? In other words, why would it ever be considered correct to write this:
… instead of this?
The first one is indistinguishable from a tie and only creates confusion or a misinterpretation, whereas the second version is crystal clear.
As a bit of an aside, you ask, “Imagine a situation involving multiple voices”—but these are multiple voices. Just to make sure we’re on the same page, I think we’re running into the issue where what music theory defines as a “voice” and what music notation software defines as a “voice” are not always one and the same. I guess you mean that this example would count as one “voice” as far as the software is concerned, even though these are actually two separate voices stemmed together? (Two notes can’t be sung simultaneously by one singer, so these are by definition two voices.)
There’s a real-life example that springs to mind: Chopin clearly differentiated between slurs and ties in his manuscripts, but the distinction can’t always be shown in standard printed notation. This is how he wrote ties, carefully connecting two note heads:
Here are two examples of slurs from his E minor Prelude; the bottom one is the one in question:
In the 1982 Henle edition, the editor interpreted this slur thus:
Later, in the 2007 Henle edition, the editor printed this, which is technically correct but looks like a tie:
Eigeldinger, in his edition for Peters, copied Chopin’s slurs extending well beyond the identical note heads, thus clearly marking them as slurs rather than ties:
This may not quite conform to modern printed notation standards, but it at least avoids the confusion between slurs and ties. (Too bad the upper voice in the left hand also consists of identical notes, so the workaround of flipping the slurs doesn’t work in this case!)
Anyway, just hoping to learn a bit more about the often confusing distinction between slurs and ties. (Let’s not even get started on Beethoven’s pairs of tied notes, which make for heated debate!)