This doesn’t exactly answer your question, but from personal experience, when I got my jabs from Walmart, the website didn’t seem to differentiate from first and second doses. That is, from the perspective of the appointment scheduling system, I seemed to get two “first” doses. And I didn’t even get a vaccination card until I specifically asked for one and they went out of their way to look me up in the system. So I don’t think there’s anything stopping me from getting another jab if I wanted it, in practice.
Similar experience—I got a J&J from a mass vaccination site in April, then a Pfizer from Walgreens a few months later. Walgreens didn’t ask me anything about whether I’d been vaccinated before.
This doesn’t exactly answer your question, but from personal experience, when I got my jabs from Walmart, the website didn’t seem to differentiate from first and second doses. That is, from the perspective of the appointment scheduling system, I seemed to get two “first” doses. And I didn’t even get a vaccination card until I specifically asked for one and they went out of their way to look me up in the system. So I don’t think there’s anything stopping me from getting another jab if I wanted it, in practice.
Similar experience—I got a J&J from a mass vaccination site in April, then a Pfizer from Walgreens a few months later. Walgreens didn’t ask me anything about whether I’d been vaccinated before.