There can be only one (or two)
Heard an unintentionally funny line on the radio this morning:
"There's only one person who can answer this, and that's y'all."
—Car Talk caller, 4 April 2010.
Of course, "y'all" can be singular in some dialects. But in this case it was clear that the caller was addressing two people, the two Car Talk guys. It seemed to me that there was even a slight hesitation before "y'all" as she realized what she was saying, but I may have read too much into it.
I don't remember for sure, but I don't think she had a Southern accent; I suspect she was using the Northerner version of "y'all," which I've been hearing more often in recent years as a disambiguating plural "you" (which is also how I use it).
It may well be that she thought of the Car Talk guys as interchangeable—I know I can't tell them apart. But I think there may've been something else going on as well:
I'm pretty sure I've heard a construction like "there's only one X, and it's Y" (with Y being a plural noun) before, may even have said it myself.
So it may be that "there's only one X" is a kind of idiom or semi-fixed phrase or exaggeration-for-effect that really just means "Y is very likely to be an X."