Verlen Kruger, who paddled 100,000+ miles, used a single-bladed paddle.
If it's so good, why not much more common? Certainly he "knew" what he was up to and had choice of paddles.
His boat design I guess is a "covered canoe" that may accomodate kneeling. But has kayak-like seat, I think...(not sure).
But double seems more secure, because one or other blade is always effectively in a brace. [An instructor once explained theory, if any blade is in the water you can't (or shouldn't need to) flip.]