Serious duplicate bug? MacOs 8.6 version.

That’s what it says in the AppleScript Finder Guide (written in 1996) and it’s what you’d expect by analogy with other AppleScript results. My point is that the result of the command is exactly what’s advertised in the Mac OS 8.6 Finder dictionary - a reference to the duplicated object(s), not to the duplicate one(s) As such, “serious duplicate bug” is not the right way to describe it.
However, without knowing why it was implemented this way, I agree with you that a reference to the duplicate(s) would be preferable.

Hope it helps!