Yes. Kes was artificially put into it and made a point of saying it happens only once.

As to them having a tendency to single births, I don't remember. There do seem to be few enough of them to indicate a population in decline, but...

If each couple can only have one child ever, their population would halve every generation, wouldn't it? That's... Isn't that wildly unsustainable, biologically?

I suppose they could represent an evolutionary dead-end...