That's going to depend on the language in the marketing agreement you signed with your Realtor. The answer to your question is in the contract you signed.
In most cases, unless you wrote in an "exclusion" for a prior viewer, most of the marketing agreements require that the seller pay a full commission, even if it's someone who looked at it before.
technically, your realtor probably isn't a dual agent... rather the buyer is "unrepresented" (as you say), but since they're unrepresented, there is no buyer's agent to pay, and according to the contract you probably signed, if there's no buyer's agent/agency to pay, they get to keep the full commission.
So while they're not really a dual agent... they do get to keep both sides of the commission.